Follow up to r77411. Patch by DieBuche

Follow up to r77411. Patch by DieBuche

1. Adds a line css to actually fix Bug 3770 (Sorry, I had forgotten to paste it
there as well)
2. Fixes a problem, where the box size would be slighty higher if the image
thumb has a height of less than 8px
3. Normalize the calculation of the vertical padding to be exactly the same one
as the horizontal padding. (This is only relevant for unusually large
galleries, eg. previously it returned a not-exactly-square box if you specified
500px as both width and height)