Ok now i am back at a pc i can help you better,
Find the file in the screenshots folder screenshots_viewer.tpl there are a couple of adjustments you need to make to the code from displaying 4 pictures wide to 3 which fits in with the skin you are using.
For me its line 65,
if ($tiles == '3') { echo '</tr>'; $tiles=0; }
default code is,
if ($tiles == '4')
Change the 4 to a 3, there are a couple of other tpl files you need to do this with as well.