Hi,
I am having the same problem, but it has only started recently.
The older pictures I have added to Cards remain in place. But ones I've added in the last few months (say 2 or 3 at a guess) stay for a few days then vanish.
To get them back, I click 'remove cover' followed by 'add cover', and then the pic returns for another few days.
The pics are important for the feel and the appeal of the Cards, so I would really appreciate a solution.