Google’s Matt Cutts assures us that using stock images on our sites doesn’t affect search results, but you should care about the type of images you’re using nonetheless.
Google’s Matt Cutts: Stock Images Do Not Impact Search Engine Rankings: “Matt’s answer was very short, he said ‘no.’ There is no positive or negative impact on your organic Web ranking if you use stock imagery versus original imagery.”
While using stock images might not negatively impact your site on a search ranking level, it can (and does) impact how users landing on your page interact with your content and navigation.
Almost as importantly in some cases, Google Image Search is a major source of traffic for many sites that use mostly original images instead of relying on stock images.
With our own clients, we’ve seen very impressive numbers from Google Image search when we’ve worked with them on custom images targeting specific keywords.