I don't believe you can upload images/files besides using SSH. You can use lighttpd to run a small webserver on your iPhone that allows you to browse files via Safari. I've only read about it and have not used it however. Good Luck!
I've not tried this, but in Cydia there's something called attachment= for milto. I'm not sure if this is quite what you're looking for, but it's the best I could find.