Using gimp, I made quite a few foregrounds and backgrounds in a day or two. The time-consuming part of the process was the actual race creation, not in making the images. It takes about 5 minutes to make the race images for one civ. And there are a couple of good downloads for logos, so you don't have to make those at all.
I think making them yourself is the way to go, because you may never be happy with the content that Stardock makes for races.
I strongly suggest you don't subscribe to any races on Steam, as it creates a lot of problems... or at least doing so created many problems for me. I also suggest you create a document that notes the name, logo, tech, trait, and color of any races you create to avoid overlap. It sucks to get 12 hours in and find out you have overlap.
edit: its also pretty easy to transfer images from GCII to GCIII and vice versa, so if you have a lot of races from II, just use something like Gimp to transfer them to III. The logos in GCII don't need to be edited at all, they are the same size.