Are you aware of the all the Cards that have been created already? There is a bunch by a bunch of different people. You can find them here:
http://www.rhinopeel.com/StatsLab/wi...Baseball_Cards
If you are looking for something more specific you should ask. The baseball card mod's aren't as easy as 1-2-3, it takes a bit of work. If you know graphics and XML you can give it a shot. I have tried and failed, which is why I appreciate the work of the guys who do make it happen. Maybe one day I will get back into, as I want to create a custom card for my league...
Hope that helps you find something 'new'.
Cheers.