For userstyles.org, click on a style's screenshot to view it at its native size (maximized [almost] up to the size of the content window) centered on a dark background.

Using the left mouse button you can
(a) click [on the image] & hold [the mouse button down] to temporarily enlarge the image,
or
(b) click & hold and then drag the mouse cursor [along with the image--release the mouse button when the mouse cursor becomes the "forbidden" icon (a circle with a slash)--only takes a few pixels worth of dragging] to enlarge the image till you click on it again.

Once you "fix" the image using method (b), for multiple screenshots you can hover the mouse cursor over the 'Before' and 'After' "tabs" (aka the screenshot captions) to switch [back and forth] between screenshots. The 'Before' screenshot will have a red border, and the 'After' screenshot will have a green border (see images below). Also, if you move the mouse cursor away from the content window, the display will revert to the 'After' screenshot [if the 'Before' screenshot had the focus]. Among other things this style can make performing pixel-by-pixel comparisons between identically-sized Before/After screenshots considerably easier.

If you want a semi-transparent background for the image(s) instead of opaque, then see the comments in the code outlining how to have that (2 steps). Be aware, though, that both images (when they're different sizes) may be visible in an overlay (but you might prefer that :D):
http://a.imageshack.us/img202/1663/usoclickscreenshotforta.png