I particularly like this article as his example of the kind of image that gets ignored is on a university website - the kind of generic attractive-students-hanging-about pic that we see on many sites.
Some types of pictures are completely ignored. This is typically the case for big feel-good images that are purely decorative.
Other types of pictures are treated as important content and scrutinized. Photos of products and real people (as opposed to stock photos of models) often fall into this category.
Photos as Web Content - article by Jakob Nielsen
UPDATE: Marketing images' negative effect (February 2011)