In the GPLv3, section 6, it says:
Basically, every time you distribute printed copies of the cards, you must also distribute the source files for those cards alongside the cards. So if you send me a deck, I should expect a CD containing the source files for all the card art, or a written offer to download it with specific instructions on how to do so.You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License
Next, the GPL says:
The GPL was written with software in mind, so this is quite difficult to interpret. Do raster images (ex .png) count as the "preferred form"? Apparently this clause was written to prevent people from sending printed copies of source code alongside a work, but I don't know how it applies to art.The “source code” for a work means the preferred form of the work for making modifications to it.
I would argue that the preferred form of a digital illustration is a layered xcf or kra file, or an svg if it's a vector graphic. I would argue that a .psd is not a preferred form because it's a proprietary format which requires proprietary software to edit. This is, at the very least, against the spirit of the GPL. I would suggest asking artists to use GIMP or Krita to create their works, otherwise people without Photoshop will be prevented from enjoying the benefits of libre art.
Finally, I've seen a few places say that all the artwork is licensed under GPLv2+, but I think this is unclear in the wtactics/art GitHub repo, which shows the license text for GPLv2 in many places while merely referencing GPLv3. I think maybe we should include both versions of the full license text, or link to both versions.
I also think it's worth considering using CC-BY-SA instead of GPL for Arcmage. It has generally the same properties as the GPL, it's compatible with the GPL (so BfW art will still work fine) but it doesn't distinguish between "source forms" and "object forms" of the work. The only additional requirement is that you prominently give credit the author of the work.