I installed Image Handler v4.3.4 on Zen Cart v1.5.5e and then wanted to install a paid gallery module as well. So I purchased and installed ZX Gallery v1.0.

Image handler v.4 and ZX Gallery v.1 have a conflict, due to the jquery file. Dr. Byte helped me fix this issue. Here is what he had to say about the confilict:
The jquery-image-hover that comes with Imagehander seems to be what was breaking your menu dropdowns, so I've disabled it by renaming the file so it never loads.
So if you are using Image Handler but also want a full gallery plugin ZC Gallery is a nice plugin. But you will need to address this issue by what was posted above. This is all I know so if you have the same issue please refer to Dr. Byte he was the one who helped me.