TwitPic - an application that allows users to post images on Twitter using mobile phones or through the twitpic website; this in turn allows the addition of image captioning/commenting by learners and assessors/verifiers/peers etc.

Note: If you want to use this you and/or learners will have to 'join' twitter as well!

See for yourself at http://twitpic.com/. 

Or take a look at an image I uploaded at http://twitpic.com/juu