If you need help learning a few interesting tricks in GIMP, I can show you a few things from my experience; but not how to make sigs in its entirety.
In my sigs that I go srs on I usually don't use C4Ds, for example, I use stocks and random pictures found off google with random filters on them, such as the one I posted a few days ago.
They make interesting effects that I can either choose to use or not use.
--that said, a good sig will take hours, less if you completely know what you're doing, what you're aiming for, or if you get lucky and it looks great before you're completely done.