I just discovered a fascinating phenomenon with certain texture(s) that I initially thought was a bug in Celestia. While trying to replace certain JPEG textures with their transparent .PNG counterparts, I found that the .PNG's would not load. After investigating the matter, I could not find any logical reason for the failure. This lead me to investigate the textures themselves. After using several different graphics programs to view the textures, I remembered that one in particular would alert me if the file extension was incorrect. I loaded the PNG into the program (which supports DDS, PNG and virtually every other extension), but again could find no reason for the load failure. I decided to RE-save the texture (a fix for problem, web, graphics I inadvertantly discovered a while back) under the PNG extension. While saving the graphic, the program gave me the option to save the original transparencies which, of course, I checked off.
Though the textures did not show signs of corruption, re-saving them to their original format did the trick and now they load properly.
The program is Irfanview. It's free and can be found here:
http://www.irfanview.com/
Post Script: The texture discrepancy may have occurred during the download process or may have originated on the server. The point is that there were no outward manifestations of it at any time. If you have a problem texture, try this and see if it works!
