Email should be plaintext
You should add a way to generate a plaintext signature that looks nice
Hosting images on dropbox might be an issue. Maybe you should use data url so the signature is "self-hosted".
What if Richard Stallman wants to contact me?
I really dislike this type of things, I prefer plain text on email, HTML adds bloat to an otherwise small email. The site, and end result, though, looks nice!
If your service becomes known at work (a university) I will have to block it; my users (faculty, that is) will swarm it like bees to a hive. :-)