I couldn't echo these words more. Not only true within the affiliate arena, but essentially any site that you have control over.
Another solultion would be to isolate two product lines, and apply image links on one, and not on the other. From there, let the raw data show what lends itself best to conversions.
The trick is to only change one element at a time, and often revert back to the original setup. Let the data come in, record it, then plan another revision based on what you know. That cycle should reveal what types of changes yield the most results for your exact audience.