Make sure you use a compressed imageformat like PNG. As cards have a lot of white and samey colours, they can be compressed very efficiently. Also try to use the smallest palette possible. A regular playing card should easily fit in 8 or 16 colours, and maybe a few more for the picture-cards like K,Q,J.
You joking?
Only 5 colours are ever needed for playing cards. Including the backing pattern on the reverse and the picture cards plus jokers.
White, Black, Red, Blue and Yellow