Hi!
I'm using the Swedish locale of Celestia 1.5 (just in case that would matter), and was just now trying to use a solar texture add-on.
However, I soon found that it never accepted my gstar.jpg add-on in the hires folder, or even in any folders in Celestia's "standard" hierarchy (i.e. not the extras one).
So I renamed all default gstar.jpg's at once, and to my surprise, the Sun still had its spots and used the standard texture! I then renamed my extras directory to something else (although I had double checked before to ensure I wasn't overriding the texture here), and still got the same result.
I then renamed the standard textures folder to take out ALL the textures in Celestia, and then sure enough the Sun lost its spots as a clear sign it lost its texture.
So it wasn't about gstar, but some other texture. After some trial and error, I saw that the Sun lose it spots when I rename the astar.jpg files!! (and yes, at the same time as the standard gstar.jpg's are located in their lores+medres directories with their proper names)
Can anyone confirm this?
Edit: Btw, my celestia.cfg is unmodified from the stock settings, with the star classification "G" supposed to use "gstar.*". This all took me by surprise too, because it doesn't look white like a class A star, but I guess it's due to the yellow hue of the atmosphere or something? It sure doesn't seem to use gstar.jpg for me at least.