Upon doing a quick Google search, I found that exact image here. The title of the picture is 'calgar', which made me think California Garter. It appears as though everybody else is correct. It is a San Francisco Garter Snake (Thamnophis sirtalis tetrataenia).
Something similar I just found is a Neon Blue California Red Sided Garter. They're stunning, and that website has lots and lots of Garter choices. Just make sure you do your research first!