I'm sorry, but this issue has absolutely nothing to do with 4images, u should contact NIS support center, not 4images support...
I've had exact same issue with NIS (note, it wasnt only happeing with 4images based sites!), and finaly I got tired of that BS and gave up, switched to another software, its just wasnt worth the trouble for me to continue suffer with NIS.
In my understanding NIS give some sort of "points" for each image (file size + dimentions + name + whatever) and if sertain number of points is reached it treat these images as ads. Well, whatever it does, I think its very not "thought through" that end-user has no control over this...