I'll be honest and say I am a little unsure if this is to do with this widget, or the widget that I have it in (Widgets on Pages) but I think it's this one as it concerns the layout of the thumbnail images.
There's a small column of white on the right hand side of the thumbnails. It doesn't matter what size it is, it's always there.
See this pic
I can't give you the web address as I'm working on it locally but I use Firebug and can see that the problem seems to be
<span class="wrap_image attachment-gallery wp-post-image">
Any help much appreciated!