Image optimization is often an overlooked part of SEO, but for some reason images often get the crap spammed out of them by SEOs trying to improve their organic page rankings. Regardless of whether you’re optimizing an image for image search or optimizing key parts of an image for optimization of the page it resides on, a more subtle approach is needed.
When you place images into a web page you have the option of adding an alt attribute. This attribute is a place to add alternative text that will be viewable when the image doesn’t display. The purpose of the alt text is to describe the image. Period.
For the most part, you don’t have to worry about your images not displaying, unless the visitor is connecting from the year 1998, they are visually impaired and using a screen reader, or you have a broken image link in place. It’s the latter two we are most concerned about.
The alt text allows these visitors to understand the visual reason and purpose for the image, even though they can’t see it. Far too often, however, SEOs stuff the alt attribute with keywords.
There is nothing wrong with adding keywords to your alt attribute. But, like all content, keywords need to be an enhancing part of the message, not a replacement (or destroyer) of the message.
I’m in favor of adding alt text to every image. But use it appropriately. There is a small bit of SEO value by optimizing your image alt tags, but focusing only on keywords instead of the message is not good optimization. Not in content, and not in alt text either.