Writing an alternative text description
Alternative text descriptions preserves meaning and helps accessibility.
All graphics need an alternative text description (ALT text) - a piece of mark-up that the browser can use if the graphic file is unavailable. This is an editorial not a design matter.
Using alternative text descriptions preserves meaning if the image is slow to load or gets lost. If the image is a link the user can follow it because the browser displays the alternative text description instead.
Using alternative text descriptions is one of the most important ways to make your site more accessible. If it's missing, people using speaking browsers hear IMAGE h t t p colon slash slash www dot intengu do co uk slash images slash bonkers dot gif or some similar twaddle, instead of a concise, informative description of the what the picture illustrates.
An alternative text description represents the meaning of the image, it doesn't describe the image.
The ALT text for this unlinked image is 'Green computer mouse'.
But…it is perfectly fair to say the mouse icon is just decoration and give it an empty ALT text. This is not the same as no ALT text (think empty box on the table v no box on the table).
For graphics used as links, ALT text is essential because the image lets the user do something.
The ALT text for this linked image is 'PC mouse deals'.
Trying to be helpful, it's easy to get carried away - 'Image of a green mouse icon, link to PC mouse deals website', 'Our logo, go to the homepage' etc.