I'm having an issue with the latest version. Images narrower than the cover are no longer centered horizontally:
I downloaded the plugin's git repository and identified the last commit that was working correctly as #2173f44dce5e30c4d195ec4158a3cd9f92518e4d ("GC: Remove draw_old.py, not needed in Cal v2+").
Unsurprisingly, the one that introduced the off-center behavior was a change to the way the "left" position variable is calculated (#1b7e5e72ab80d5c82884ad206faf64a63f5d5548 "GC: Fix for border causing img to move horz."), mentioned a few posts earlier in this thread.
I'm running this under Calibre v3.48