The border around the currently-selected thumbnail image needs to be bolder. The current 2-pixel border is hard to distinguish from the 1-pixel "frame" around all the other ones ... at least not "at a glance". I'd like to see a 3-pixel border ... and maybe even a colored border just for the selected image.
(I've used the word 'shortcoming' reluctantly ... because, overall, I find FreeCommander to be a real "gem" with respect to the level of tailoring that can be done to closely match a user's needs ... whether those needs are simple or complex. This improvement would be icing on what is already a very good cake.)