One thing you could do is try to get a more recent version of
GD, which handles PNG and JPEG. I realize that recent versions
do not support GIF, but that's okay, because you can just
convert all of your GIF images to PNG. :)
If that doesn't really appeal to you, you could use the NETPBM
tools to convert and resize, etc. merlyn has a column on using NETPBM tools to create thumbnails, which may
be what you need. If the NETPBM tools aren't already installed
on your machine, you can get them from ftp://ftp.x.org/contrib/utilities/netpbm-1mar1994.p1.tar.gz (per the link in
merlyn's article).