As they appear now, your images all appear to be like photographic negatives. Can you instead make them appear as positive images? I think it would be more attractive.
While this is never mandatory, it's usually a good idea to keep the back design two-way - the same when rotated 180 degrees. Either that, or a one-way design that's more obvious - say, one Mandrake image taking up the entire back. A subtle, one-way design can be used by cheaters to rig a deck - for example, turn high-value cards one way and low-value cards the other when shuffling and now you know who's getting which value category.