This is actually an undocumented feature since 2.0 firmware, but it's really easy:
And that's about it. Press E-mail Photo and send it to submit@iznap.com.
submit@iznap.com
The content of the "subject field" will be the title of the screenshot.
You can tag your iPhone screenshot by writing descriptive words separated by space in the "content area" above the screenshot. You can combine words in one tag by putting quotation around it like this: "damage control".