Thanks for the comments

I made the textures with Gimp 2.2.7, though the technique should work on any version of the software. I started with a solid orange background, then on a new layer rendered clouds (solid noise) with a detail level of 15, turbulence, and seamlesness turned on, and set the layer to "grain extract", which left me with the basic lava texture. I then took it to Photoplus 6 (another free editor), because I like its waves filter, then brought it back to Gimp and made it seamless again. The second texture is the same as the first one, with a little gamma and contrast correction.

It shouldn't be too hard to modify the process slightly and also get a nice piece of lightning, too