In ImageGlass, "Auto-zoom" is:
- show image at 100% if smaller than window;
- otherwise show as scale-to-fit
If you want something like "always show images at 100%, even if larger than the window", then do the following:
1. Select "Actual Size" (Alt+0 [zero])
2. Select "Lock zoom ratio" (Ctrl+L)
If I've misunderstood your request, please clarify.
Enjoy!