From the Flickr description:

A juvenile emerald tree boa serves as an unwitting perch to a recently awakened red eyed tree frog after a misting. The snake and several frogs cohabitated for quite some time without incident. Perhaps due to an unpalatable flavor brightly colored frogs often advertise.

It should be noted that the red-eyed tree frog is not actually venomous. The snake likely would not know that, so if the snake is able to detect the color of the frog, this explanation might be correct. Still, I wonder if there’s anything more to it?

Via Tom’s Flickr photostream.
Thanks Kimberly!