A suggestion (please correct me if there IS a way to go about this): Maybe add a button in the 'Edit' menu to change the thumbnail for tabs (sub-gallery)? Or better yet, an entire section in Settings dedicated to tab management/customization? The only way I can go about changing a sub-galleries thumbnail is by going into each individual sub-gallery, uploading an image into it with the 'Set as avatar' box selected, then deleting it. It's very tedious and time consuming, especially since each of my ~40 characters have at least 3 sub-galleries each. I barely use the forums because there's so much things to look at and it freaks me out, but this is the one time I found something worthy to suggest on this thread. I'm sorry if this was already suggested or is already being worked on.
edit: I'm stupid ,':-)