As it turns out, the great thing about jpegs is that they use lossey compression to squash the image down to a tiny size. This is great for pics on the web and ok for photos, but not too helpfull for texturemaps.
What you want to use is (a) a format that uses lossless compression or (b) an uncompressed format, with (a) being the prefferential one here.
You write that you're using photoshop, then just save em as TGAs of TIFFs and you'll be fine. Unfortunately, this means the file is _much_ bigger. But it's a necessary evil.
Much like DV-AVI vs MPEG....
And of course, keep a copy with all the layer info saved in .psd format.
Hope it helps,