Just read this discussion about using someone else's design as signature. Although its a nice idea to ask permission, I feel you can use those signatures. First of all, signatures are made from internet images which are free for all. There is no image that is used for signatures that is exclusively copyighted to the individual that designs the signature. If you start asking for copyrights on your designs, you are basically asking for copyrights on those original images which you use for the signature and thats an offense (those are free for all, and you are calling it your own). So if you want people to ask permission on using your designs, then use images in your design that are completely your own (not free for all or copyrighted images).