Porn is definitely not allowed in sigs, and if you see something thats going to far please report it to us, but that doenst mean to say that a picture of a girl in a signature isn't allowed.
If your work environment is particulary strict or it would just be awkward to have the pics on your screen all the time you can disable signatures by going into "settings", "edit settings" and scroll down to the check box with signatures in it.
We also have a fully work safe way of viewing the forums while retaining all functionality, refer to this thread for info on how to enable that mode:
http://www.e90post.com/forums/showthread.php?t=9046
Hope it helps! let me know if there are particular problems