For example, lets say a T-Shirt site, they usually have a bunch of designs or products related to a certain content. If your PHP code is generating these images or grabbing them from a Database (files names usually come from a Database), you probably want to take advantage of that by adding to the ALT tag the title of the product. This also accomplish two things: a) If user has a slow down in his connection he will see the product name until the image loads, making it easy for them to recognize what the product image is about, and of course b) to help with your positioning in the search results.

So, just make sure the code looks like this < img src = “/folder/your-image-name.jpg” alt = “Talk nerdy to me” >. Oh, almost forgot, to those who love Internet Explorer, you might want to include the TITLE tag as well.