The problem with your suggestions in your reply to the OP, is that these options are global, meaning they act for both normal screen and fullscreen. So when we return from fullscreen to normal screen, toolbar and other UI elements are still disabled and you have to manually re-enable them.
I like having the toolbar and thumbnail strip when using the program in normal screen, but I don't like them when using it in fullscreen, where I prefer a clean, uncluttered view of the picture filling up my entire monitor (other than letterboxing/pillarboxing for aspect ratio correction) and nothing else. Having to manually toggle them on/off every time we switch in and out of fullscreen is an usability nightmare, it's just not an acceptable solution.
Normal and fullscreen modes should have separate settings for the display of UI elements such as toolbar, thumnail strip and background (and of course, these settings should be saved and used as default every time we run the program). This is one of the most important issues that keep me from adopting ImageGlass as my primary picture viewing program.
So, I second OP's issues with this and leave my ideas as a suggestion / feature request. Thank you.