Requiring admin rights can be requested by including the proper manifest setting, without admin rights it now just throws an errormessage.

What I read from the OP is that not the smallest file is the issue, but the fact that vBulletin displays the jpg's larger then png's. And in the end that's just a setting/style fix on the vBulletin side...

From that startingpoint, the discussion on jpg vs. png, it comes down to the actual use, seeing that jpg is crap on fine details, but quite alright on photos, where png is very good in fine details, but also good in photos, with the side note that those png-photo files seem to be a tad larger than their jpg counterparts.

Is another application catching the Shift PrSc hotkey, effectively disabling ScreenShot Captor?
DropBox/OneDrive (don't recall exactly which one, maybe both) seem to do that, but can usually be fixed by configuring them not to capture screenshots.