I'm not even sure what texture we are talking about... the nebula backgrounds?
@Old Dragon:
Just increasing the image size would be rather pointless, if there's no additional detail to display. Instead of a small texture you'd now have a big blurry one. You could use some sharpen filters on it, but the results won't be all that great either... while you could enhance the features and details of the original low res texture that way, you can't improve the quality by much. It's also easy to oversharpen, introducing some ugly visual artifacts and glitches on the texture.
The only good method would be to create new replacement textures from scratch, but in a higher resolution than the originals. Or you could increase the image size of the original and then paint new details into the now blurry texture.
