You don't need to edit image CSS. You need to edit your template's CSS file to make the center column wider on your product info pages. Right now...it's too narrow to accommodate the size image you want to have displayed.
I can't tell you what that file is or where to find it because I don't know if you're using a custom template, if you have set up custom template folders, or what you've named your custom files.
Suggest you download and install the Firefox browser and the Firebug addon for the browser.
Then use this tool to help you figure out what part of your template's CSS needs to be edited to display the larger images.
But you also need to understand that you need to start with the size image you want displayed...meaning you must upload that size image....neither your ZC image settings, nor your IH3 image settings will enlarge smaller images. These settings only make images smaller (which is why you always upload your largest image and let IH size them down where appropriate - read the readme file that came with your IH3 download package).
Good luck and happy Zenning
