What about using a POP account? Can I use this too? I am jailbroken.Jailbreak your phone and install Mail Enhancer Pro from Cydia. It allows you to use HTML in your signature so all you have to do is have your logo hosted on a media hosting site like imageshack, photo bucket, etc. Then copy the direct link to the image in your sig using HTML.
This is what my sig looks like with an image:
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