RedHat (at least, the 4.1 release that I have) comes with a package
called ImageMagic which has a pant-load of converters. And a nice
interface to them - simply say "convert src dest" and it figures
out what to do by the extensions.
I think that it is freely available, but I don't know where you'd
find it.
Search for "netpbm" or "pbmplus".
If you can't find it, I can send you an attachment of a precompiled package
I found a long time ago.
I absolutely prefer it to ImageAlchemy and similar packages since they rely
on a lot of libraries you have to install by yourself.
/Paw Mullit