If you are going to do a lot of hand painting, something I personally like doing too (in conjunction with 3d painting tools), best to use a seamless pattern fill of a skin tile as your base fill. In  photoshop you can scale the pattern filling, I imagine you can do the same in Gimp. It might seem, in theory, that using a pattern fill would lead to some obvious repetitive appearance but I've found that it really depends on the tile. Oddly enough suitable skin tiles are difficult to find... this is actually a very good one

https://www.renderosity.com/mod/bcs/skinsource---artist-tool/88029

Also you'll find that baking cavity or AO maps and using them as a multiply layer, or other blending modes, of varying intensity, and judicious use of clone stamping and noise based brushes (and erasers!) will be very useful... I personally have dozens of skin and detail brush sets for Photoshop that I've also taken the time to convert to mudbox that I use quite a bit.

Hand painting things such as tongue details, wrinkles on knuckles, etc is tricky. You could patchwork photo sources for this. Make sure not to make the wrinkle details too dark, most photo resources will cause this. Knock back the opacity a bit and rely on bump for the "shading". Also sculpting these details and using generated displacement maps as a multiply layer or cavity map is helpful for these kinds of details.