Selected answers from the Dumb SEO Questions Facebook & G+ community.
Gabriel Partenie: Hi, Does an external backlink that use image as anchor have any SEO value? Thanks in advance!
Edwin Jonk: Yes, the alt text can be seen as the anchor text.
Shawn Barrington: It can if the image is associated with some great content around it. However using proper alt attributes would be the beat way to get the most benefit from a image based link.
Greg Baka: ...and even if there is no alt text or title on the image, you can still get some value if the topic of the page the linked image is on is closely related to the topic of your page. (based on what I have read)