I use Trello to organize job and service calls, and use my customers' company logos as the card covers for quick and easy identification. When creating a new card, I'd like to be able to quickly choose from a collection of uploaded/always available photos to use as the card's cover.
Currently, what I do when creating a new card is go find an old card related to that customer, open the image, right click to copy it, then go back to the newer card and past it in. It's not terrible but it does interrupt the flow.
I think this would also have the benefit of saving space so the same image isn't uploaded 300 times and instead is just referenced.