As RyogaU (and others) have pointed out, this addon is currently broken when viewing large, centered images, but the fix he recommended did work for me and now it works without any issues. For anyone else that wishes to try it, you can find the bootstrap.js file in Users\(your name)\AppData\Roaming\Mozilla\Firefox\Profiles\either folder (check both)\Extensions\olddefaultimagestyle@etcetera. I don't know if the profile folder names are the same for everyone or not, hence why I didn't specify which one. To add his fix, open the extension file with WinRAR or any other archive program, then open the bootstrap.js file in Notepad++ and follow his instructions. Huge thanks to you for sharing it RyogaU, but I thought I'd help others figure out where the file is since I didn't know myself until now.
As for the addon itself, it's fantastic. I use the NASA Night Launch theme (with a bit of jury rigging to get it working with Firefox 47) and previously it had a dark grey background for images, but even after getting it working on 47 it lost that feature and the background defaulted to white, which I hate. Thanks to this plugin, I was able to get it exactly how it was previously, and I haven't seen any other addons that let you customize the background for image tabs. There are a few for customizing the about:home page on new tabs, but those don't apply to image tabs for whatever reason. Thanks to Dagger for creating this, and again, a huge thanks to RyogaU for providing a fix for newer versions of Firefox.