When adding an image to a web page its good to include a short piece of text in the Alt and Title tags of that image. This helps Google and other search engines understand what your images are about and it also helps visually impaired people or people using screen readers or where the image simply does not display correctly understand what images are about on your web page. You might also like to see Googles explanation about Alt tags.
How to add the the text attributes to your images
Matt Cutts discusses the alt attribute
