Hi Leah,
I just made it configurable in my copy from Web.config, this will be in svn later tonight. After getting the update you can add a setting in user.config for
DefaultResizeBackgroundColor
It should be a standard named color like White or Black or anything from the NamedColors enum, if something incorrect is entered I think it will use Black. The setting defaults to White.
I tried just commenting out that line but the result is then a black border in the thumbnail, so that line is really trying to get rid of a border but it uses white which blends in more often.
Best,
Joe