optimal resolution of your artwork - enough to be estimated by people, not enough to be easily printed in good quaility;
That doesn't always work. Because a playing card is so tiny, you don't require very high resolution images to print it.
There's one trick I heard of that seems to work, at least as far as proving you made yours first. Go the copyright route, definitely, but also make a copy of your work and mail it to yourself via Registered Mail. Don't open it unless it's needed as evidence in a court case. The sealed envelope and the postmark would be adequate proof that you completed your artwork by the date on the postmark and the Post Office keeps records of anything sent Registered Mail - further proof that the mark is genuine and not forged.
Bear in mind that this would hold up well in a US court - pursuing cases elsewhere would be hit or miss and there are corners of the world that pay little more than lip service to copyrights.