The pixel dimensions of an image are the number of coloured dots or pixels horizontally and vertically that form the image.  1024 x 768 means there are 1024 dots horizontally and 768 vertically – a total of 1024 x 768 = 786,432 pixels.

This is the most important property of an image as it determines how large the image will be when displayed on a screen and when printed on paper.

The resolution of an image is a measure of how close together its pixels are when displayed or printed.  This effects how large and how sharp the image will appear on screen and when printed. A 72dpi (dots per inch) image means each horizontal and vertical inch of the image will be made up of 72 dots whereas a 300dpi image means each inch has 300 dots – much closer together.

A 200 x 200 pixel image displayed on screen at 72dpi appears 2.8in (inches) wide and high (200 ÷ 72).

The same image (i.e. 200 x 200 pixels) printed on paper at 300dpi is only 0.67in wide and high (200 ÷ 300).

On a computer screen (traditionally with a resolution of 72dpi) a 72dpi image will look sharp as the adjacent pixels glow and this bridges the gaps between them. But if the image is printed on paper at 72dpi the gaps between the dots are apparent and the image appears out of focus (‘pixellated’). 

This explains why images are printed at 300dpi; the gaps between the dots are not so apparent. The reason 300dpi images are not used on websites is that they will have large file sizes which will increase the time the page takes to load and not look any sharper than a 72dpi image.

The size of a printed image is determined by its pixel dimensions and its resolution.

If an image is to be printed it needs to have a resolution of 300dpi otherwise it will appeared blurred.  An image that has been converted to 300dpi has that information encoded in its file as an instruction that is read by the printer. Just changing the resolution of an image (e.g. from 72dpi to 300dpi) does not change the number of pixels – only how closely together they will be printed.

This means that the maximum size in inches of a printed image is set by the number of pixels it contains (pixels ÷ 300).  If you try to print it larger than this it will be blurred as you will be forcing the dots to be further apart.

You can use a specialist graphics program, such as Photoshop, to force an image to print larger than that set by its pixel dimensions. The software will ‘resample’ the image and add extra pixels to keep the dpi at 300. The software is in effect ‘guessing’ what colour the inserted pixels should be which means the image won’t look as blurred as it would otherwise but may not be completely sharp.

The file format refers to how an image has been encoded as digital information.  Jpg and gif encoding produces much smaller files than the others but they tend to be lower quality images.  Smaller files make it easier to email images and when included on website pages make the pages load more quickly.

This refers to the amount of space needed to store the file. The smaller the file size of an image the less space it will take up on your hard drive or mobile phone and the quicker it will take to email as an attachment.

When you save an image as a jpg using a graphics program you are asked what quality you want to save it at.

As the program saves the image it analyses it and encodes groups of pixels using approximations rather than saving every pixel as is. For example it may treat pixels which have a very similar red colour as having the same colour and then encode them in the file as, say, 10 pixels of a single red colour rather than have a different code for the actual colour of each individual pixel. This reduces the file size.

When the jpg is then displayed or printed, the original pixel dimensions are re-created using the procedures in reverse.

The ‘quality’ setting determines the level of compression – that is, the extent to which the coloured pixels are treated as the same – and hence the file size. In a low quality jpg, a wide ranges of say, reds, are treated as the same colour when encoded. This will reduce the quality of the image when it is re-created but also reduce the file size further.

How to change just the Pixel Dimensions of an image

It is advisable to only reduce the number of pixels in an image as increasing them will usually significantly reduce the image’s sharpness. If the number of pixels in your image is insufficient to produce the size of image you want, then you should re-photograph the object with your camera set to a higher quality.

To change the pixel dimensions of an image:

1. Open the image resize tool and check the existing width/height dimensions (Note: pixels and points are the same unit)
2. If you want the image to have smaller dimensions, make sure Resample is selected and change the Width/Height to the dimensions required. 
3. If you want the image to have larger dimensions ONLY RISK a 10 – 15% SIZE INCREASE or else the image quality will be reduced significantly.  Make sure Resample is selected and change the Width/Height to the dimensions required.  
4. Click Ok and ‘Save as…’ your image (using a new name so you keep the original)

How to change just the Resolution (dpi) of an image

To change the resolution of an image, such as changing it from 72 dpi (images produced by most cameras) to 300 dpi (for printing):

1. Open the resize tool and UNTICK Resample.
2. Change the Resolution to 300 dpi.
3. Now check what size the image will print at by changing the units of Width and Height from Points (same as Pixels) to Inches or Centimetres

How to change just the printed Dimensions of an image

If you require an image to be printed at a specific size and assuming it already has a resolution of 300 dpi, then:

1. Open the resize tool and check the existing width/height dimensions by changing the width/height units to the ones required (e.g. inches or centimetres)
2. If you want the printed image to be a different size, TICK Resample and change the Width/Height to the size required.   If the image is to be smaller, no problem. BUT ONLY RISK A 10 – 15% SIZE INCREASE or else the image quality will be reduced significantly.
3. Click Ok and ‘Save as …’ your image.

Changing the Resolution and Dimensions at the same time

You can change an image’s resolution and dimensions at the same time using the image resize tool if you know that just changing the dpi produces an image that is too large.  

But always check that you won’t be enlarging the image by first de-selecting Resample, then changing the resolution and seeing what print dimensions (inches or cm) will be produced.

Then ensure Resample is selected, reduce the dimensions, and click ‘OK’.

If an image is saved as a .tif (or is a RAW image off a camera) the file size is determined solely by the total number of pixels in the image. Each pixel requires 3 bytes of file size to encode it.

So if you have a 1,000 x 1,000 px image, the file size will be 1000 x 1000 x 3 = 3,000,000 bytes.  

Since 1 kilobytes (KB) = 1,000 bytes and 1 megabyte = 1,000 KB,  3,000,000 bytes can be expressed as 3,000KB or  3MB.

If you saved a 1,000 x 1,000 px image (this would a 3000 KB tif) as a jpg at 60% ‘quality’ it would only be approximately 300 KB – i.e. about 10% of the size of the .tif.

A jpg file size is also effected by the amount of detail in the image as more complex images require more complex encoding. An image with a few large areas of the same colour will be much smaller as a jpg file than an image with a large amount of fine detail – even if saved at the same jpg quality.

So if you need to have a file size with a minimum number of KB or MB, and if the pixel dimensions are fixed, the only way to increase the file size of a jpg is to increase the quality it is saved at.