An alt-tag is the alternative text that is connected to a picture/graphic on a web page. This tag is exposed when someone rolls their mouse over the picture/graphic or they provide text when a photo cannot be displayed. The alt-tag should do two things:

• Express what the picture is about
• Contain a keyword/keyword phrase (no stuffing!)

Alt Tags can:

• Give a destination of a hyperlinked image
• Help people with various disabilities
• Give important information to people that  turn graphics off and those who can only surf the Web with text-only browsers
• Help people find the destination they need when viewing a site over a slow connection. The text can allow visitors to make navigation choices before graphics are displayed

When spiders scan your web site they are looking at the source page, so they are not going to see the pictures on your site, but they can see what you name the pictures.
If your site is about Sarasota lawn care services and you have a picture of your logo you do not want to label that photo “our logo”. You want to name it “Sarasota lawn care services”. The search engines are looking at the coding on your source page and the keywords within the coding. Naming your images a keyword phrase helps your optimization because it gives you another opportunity to support your optimization strategy.