while trying to make out of prettyPhoto js overlay a wp-plugin which also should work with nextgen i noticed a problem with the a and a:hover class next css entry for the next-previous arrows on the pages. Inside pretty photo there is the same definition for a next an this seems to get a little bit spoiled by yours as the div is inside your div i guess. Would it be possible and maybe generally useful to use more specific definitions for such classes e.g. a class="ngg_next" instead of a class="next" as output in nggallery.lib.php and accordingly defined in the related css files? In this case this solved my problem.