You've asked, we've listened - you can now choose an image to upload as a thumbnail that's displayed on your costume list. If you don't choose to upload one, it'll pick the last photo or progress image you added to that costume. You can add and edit this thumbnail via the costume info section.
The size for the thumbnail is 600px width by 400px height if you want to be pixel perfect, anything larger than this will be automatically cropped to these dimensions.
This thumbnail can be of your costume, reference artwork or a custom design - it's up to you. Let us know how you get on and if you have any issues with the feature, raise a Chatter or drop us a line at email@example.com.