A new mashup, Flickr2Twitter, could draw new blood for both social media websites.

Flickr has announced its integrated photo sharing service, Flickr2Twitter, is now live. The service will allow existing Flickr users to tweet content directly to their Twitter streams, using a shorter Flickr URL (similar to existing sites such as TinyURL).

Users will also be able to tweet Flickr photos straight from email using special ‘2twitter’ email addresses, which send an automatic tweet.

Although there are similar services (eg. TwitPic), Flickr2Twitter aims to lower entry barriers for Flickr users cross-posting photographs.

The partnership could potentially wound TwitPic’s status as the most popular Twitter photo client.

In related news, Apple’s iPhone is moving in on Canon’s Digital Rebel XTi as the most popular camera used on Flickr. Flickr statistics say about 6,000 users upload from a Digital Rebel, while iPhone is closing the gap at 4,000.

Since the launch of Flickr there has been an inverse relationship between the two.