GPL (FDL as well) license is pretty straightforward. You have to have some freedoms provided, period. This stuff applies to released PDF as well - basically you have to provide the graphics and working files (tex-like, scribus, whatever) in the source form, this means with all the files needed to recreate or change whatever is GPLed. FDL licensing is incompatible with what you have released.
Sorting this mess is easy, as you can simply take care for each piece independently - providing information required for every distinct part of the game. Releasing whole code tree with license and authorship information wherever needed and provide information about how to look for the licensing/authorship information as well. And you can release text on your cards on FDL, but can not relicense works in the way that violates author's will or good name.
Lossless quality file in target resolution (.bmp, .png, most of .tiff files) is the source form in case of GPLed graphics. Not a hires layered file, we would all gladly be given, really. Its like asking for oil painting that is still wet, or asking for butterfly that is on the photo plus raw camera file, or asking musician to release his sample collection and give you brand of bass strings and mic used for the recording. GPLing artwork IS ridiculous, but it sometimes happen (mostly due to people holding CC licensed artwork from getting into linux repos and following GPL virus pandemia), so this is how and why the problem is solved. Once again: source = minimum used to (re-)create exactly the same product or modify it without loosing quality. And if a person can't edit lossless file as he/she wishes, or want to abuse it, like when printing big poster out of shitty graphics, its because lack of craftsmanship or "know-howness" on his/her side.
Its not that I like content released in the way described, but this is how the world works, sadly.