Transparency isn't working

I have two side textures (grass borders, to be more specific) and for some reason, even when I outline the part that's supposed to be invisible with index 247 cyan (from the image on this wiki page: http://wiki.srb2.org/wiki/Palette), when I load it into SlumpEd and convert it to Doom GFX, it seem to reconvert the cyan into another one (the converted cyan looks like 220 index on that image. Also, I read the wiki article which says I have to use index 247 cyan specifically for SlumpEd or else it won't consider it transparent, and since that's exactly what I'm doing, I don't know) Why does this keep happening!? I've had this problem for so long (in fact, I may have made a topic on this already, but I don't think so)! Help?

And I'm ignoring you if your answer is to use something other than SlumpEd (unless it just plain doesn't work at all).
The imported grass border:

And what happens when I convert it:

This also happens when the files are in any BMP format.

I don't see your problem, PM me the picture and I'll try to find what the problem is.