With the .htaccess tag Drums posted you can still use options on the end of the url. You just need to put somthing like null=.gif and the end to fool the forums checking code.
This is my normal url to my sig
http://www.crusadersvalorous.org/lan.../sig/toran.gif
and here is one I use for soe restricted site
http://www.crusadersvalorous.org/lan...=soe&null=.gif
and here is one I use sites that have odd ball size restriction
http://www.crusadersvalorous.org/lan...=300&null=.gif
Yes I am still working on the transparency when the image gets resized:/ Other then that it works pritty good.