I am working on a couple of euro-style strategy card games at the moment. The first is a business start-up game, and the second is a space-themed game loosely based on my experiences playing the Traveler role-playing game years ago. A good stock image account (I use Shutterstock) gets me everything in terms of card images I need for the first game, but royalty-free space images are harder. However, it is actually possible to start with prosaic industrial and other images and hack them to look futuristic, but it takes some work.
So I have been working on Photoshop skills. If I could digitally paint, I would paint beautiful concept art, but I cannot. So my Photoshop training has focused not on painting per se but on hacking images together and overlaying effects. A LOT of the work is learning to do selections well to mash up images and then overlaying a few effects. I can make a really good laser beam now, for example. Take a modern weapon, have a laser beam come out, wala a pretty functional sci fi gun (Don't believe me? Look at the Death Star Turrets in the original Star Wars movie and tell me those aren't essentially current-era battleship turrets with green and red light coming out). I wrote earlier about the lessons I followed in making custom planets.
As an example, here is the lesson I did last night. It is not production value because it started with a low-res iPhone photo my daughter sent me and as you can tell from the edges and especially the hair, I did not spend much effort getting the edge selection just right. But my daughter liked being a cyborg:
She has dark brown hair and dark brown eyes, so she is not the ideal model for this because those are hard to colorize well. Blondes may or may not have more fun, but they are much easier to colorize. The downsize of the exercise is that she loved the hair and now wants to color it that way for real.