There is a fair bit of graphic design in the cards, but there's also a lot of technology. The final card images are built by a series of custom pieces of software, building up from the raw elements to the final designs.
The content begins as a large 3D environment in Blender with all the elements in it. A separate file defines each element, and the values it can display.
The definition file is processed by the distribution planner, which distributes all the elements among the cards. It tries to make sure that roughly the same number and total size of elements appears on each card. It also plans which values should appear on each card, for each element, so that all the probability distributions are correct.
The distribution plan is then passed to the packer, which lays out the elements on the card. This is like virtually rolling the dice onto the table, except that the results of each roll are known in advance. Some elements can go anywhere, others have to be in certain places (the poker chip, for example, goes near the top, the cross on the map goes near the middle).
The distribution and packing plan are then sent to the a program that controls the 3D software. It lays out the elements in the correct place in 3D space, sets the lighting and the camera, and renders the image.
It takes about five minutes to draw each card. The 3D images are then passed to a final piece of software, which adds the border, the card suit and value. This software outputs a proof image, a CMYK print-ready image, and adds the card to a PDF suitable for proof printing on a color laser printer.
A separate piece of software generates the card back images.
All in all it takes about half a day to produce a new set of cards. I can use this to produce custom decks (with the same elements on them, but in different places and with different values on each card.