Let's look at my skins directory, shall we?

See all the directories? You see some like "c202" without extensions -- these are the defaults. Never screw with your defaults, for 2 reasons. 1) you need them, unedited/unaltered, if you need to revert, start again, start a new skin, etc and 2) these don't show up as skins anyway, because they don't have extensions.
Now let's look at c202_3 (there were 2 c202 skins already, and I still wanted to look at those to compare, so I chose the 3rd slot. If you chose 1 or 2, you will overwrite the other skins in the drop-box -- they're not gone they just won't be accessible offline).

Now what have we got? The basics are the bitmaps. The c2 has 2 files because it's older. If I only have one of the files, the other will display as the default.
The c202.txt file has the name in it. If you do not have this file, or if it is blank, the option in the skins drop-down menu will simply list "c202_3". It will be the directory name.
Now, the material is from the default directory (another reason not to screw with the defaults, so you can pull up the material.txt files). [EDIT: I just realized the C2 doesn't have a material.txt file, but most others do, so I'll leave this in here]
If I make a bitmap that's not 8-bit, 256-color, and put it in here, it will NOT read the bitmap, thus allowing me to choose this skin, but displaying only the defaults for that bitmap (it will let me select it, though).
I think I've made it as clear as I could. If you do this and it doesn't work, it's the way you're saving the file. I'll wait to see a response before going from here.