I don't think it's that simple. When a user goes back to image search results, is it because they are unsatisfied with the result they clicked on, or are they looking for even more, hoping to compare? And when a user closes out instead of doing back, is it because they got what they wanted or because the results were so garbage they gave up? Based on my own usage of image search results I don't think you would be able to tell when I was happy with the results or when I wasn't. But you can probably tell that my usage of image search went down when it started getting shitty due to this spam.