The side effect of this is that, overall, your "organic" traffic might seem to perform worse vs your Adwords campaigns, since people looking for images are usually lower quality visitors.

I wonder if this is just a coincidence.

@michokest yes, that happens too. For me the biggest issue was thinking that we were doing great at some keywords with SEO and it turned out it was just Google Images. Certainly, people coming through Google Images doesn't convert at all.

After my "hack" at least we know :)