Currently the background of all framed images is grey (which is only visible for transparent images). On English Wikipedia this is overridden in MediaWiki:Common.css to make it white, but this isn't the case on other wikis.
English Wikipedia:
Spanish Wikipedia:
The CSS used on English Wikipedia is:
div.thumb .thumbimage { background-color: #fff; }
Something similar to this should probably be added to the html .thumbimage rule in /resources/src/mediawiki.skinning/content.css.