Yes you kinda of can!
I did it with my gmail account, so if you have another one than that ... try to do the same!
1 - have yourself a canned response with the wanted signature and image in it.
2 - send yourself an empty email (nothing else than the signature and picture) that you can open on your iphone
3 - copy the entire signature
4 - paste it in your signature area in your settings (settings, mail, signature .... )
5 - that's it!
If it's a simple signature it works, but i never tried to have or make a very fancy signature!
It worked for me and my associate for the past 6 months!!
Hope it works for you!