SEO: Google Image Search
There’s more to the modern Internet that just text, people are typing in keywords for images too. If you stick to accessibility guidelines your images will be more accessible to a wider audience and as an added bonus index-able with Google and other search engines.
I noticed recently that some of my visitors are coming from Google image search and more impressively they find the image they desire and often are return visitors too.
If you never had a convincing argument to make your clients use appropriate naming conventions and alternate text attributes, you do now, SEO is an easier case to argue than accessibility.
Image Alt Attributes
The ALT attribute should describe the image so that a non visual user can interpret them, chances are the image reflects the content of the page anyway and therefore probably will contain the correct page keyword in the ALT attribute.
Image Naming Conventions
I forget what I was taught in graphic design, I also doubt WCAG covers the naming convention of image files for accessibility’s sake, but for easy file management I use a descriptive naming convention that explains what is happening the the image: cat-sat-on-mat.jpg, as the image supports the content of the page chances are it will also contain your web page keyword.