Are opening a new window? Just as what you mention about FF above.
Watermark doesn't work using IH2. So forget about that.
I would do testing on IE, FF and maybe Chrome are the mostly used browsers. Safari if you have to I guess.
I suggest you look into your image sizing in the admin of your Zen Cart. Might be something astray here. Otherwise it could be a image mod problem?
Something is not working right (corrupt) or your measurement settings in admin are wrong.
admin>configuration>images
admin > Configuration > fual Slimbox
Admin> configuration > AJAX Image Swapper
etc..
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