There's been a proliferation of photo sharing apps tied to Twitter , including TwitPic, Twitxr ( review ), and Yfrog ( review ), giving users a vast amount of choice when it comes to image sharing on the popular micro-blogging service. But TwitPic seems to have emerged as the leader of the pack. The service took the top spot on our list of the most popular Twitter applications according to Compete and was in the top ten of Twitter clients according to TwitStat. Two more competitors to TwitPic have emerged. TweetPhoto and Pixim are both photo sharing applications attempting to challenge its dominance, so we took a closer look.