The new KS3 National Curriculum for Computing requires you to consider how different types of information are stored inside the computer. This page demonstrates how images are stored as numbers. Click on a pixel in the grid and use the controls to adjust the colour. Each pixel in the image is stored as three numbers - the amount of red, green and blue - and these can be seen to the left of the image.
This is only an 8 x 8 grid - 64 pixels - but even this image is stored as 64 x 3 = 192 numbers. Most images are made up of many more smaller pixels, which is why picture files can easily take up several megabytes of storage space. The images in the navigation bar to the left, for example are 40 x 36 pixels and would be represented by 40 x 36 x 3 = 4320 numbers.
Compression techniques, such as those used by jpeg, GIF and PNG files can be used to make the files smaller.
With HTML colours used in web-pages, the three numbers are combined and represented using hexadecimal - click here for details.
There is a similar page that demonstrates how a characters can be created. You can use the Metadator to see the importance of metadata in image files, or see what happens when you alter the colour values on the Image Manipulator page.