I realized while reading Second Life: The Official Guide this morning that I can stretch one dimension of a texture to fit the factor-of-two square for uploading, and then take care of it in the object texture tiling controls. (Although the stretching/compressing effects of either The Gimp or SL may end up having a negative effect on the final image.)
And I really need to find out how large a surface in SL a 512pix will adequately cover.
(The book, by the way, is pretty good. It skims over a few things, but it provides a comprehensive overview so far. I’m just starting the chapter on LSL.)