since the site has a white background, a quick workaround could be to save the pngs as jpeg with white background?
this thread mentions a possible solution:
- wordpress dot org/support/topic/plugin-nextgen-gallery-problems-with-png-transparency-thumbnails?replies=9
this has an unpdated solution:
- wordpress dot org/support/topic/plugin-nextgen-gallery-png-thumbnails-with-transparancy-problem-1?replies=5
and this thread suggests saving as 24bit png's rather than 8bit
- wordpress dot org/support/topic/nextgen-gallery-transparent-images-showing-black-background-with-resize?replies=8#post-1921440
ps cant post all links as dont have enough rep :-(
if an admin can amend i'd be grateful