Well, this challenge was certainly more difficult than I anticipated. Nonetheless, several people did find the frog in the image above. Below, I’ve outlined it.
Now that you know it’s there, I bet you can’t look at the image without it standing out.
Andrea J. went on to suggest it might be a Leptodactylid. I agree. Here’s a close crop from the photo above.
I picked up this book while in Costa Rica:
by Twan Leenders
According to that book, all Costa Rican leptodactylids lack webbing between their fingers. In the crop above, I don’t see any webbing. There are only three genera in Costa Rica. One genus has only large species, and this one is small. Another has only a single species, easily dismissed because it has extremely warty skin.
So by deduction this must be an Eleutherodactylus species. There are 40 highly variable species in that genus that the book calls a “taxonomic nightmare”, so I won’t speculate further on the species. I might even be wrong about the family :). See comments below… I was fooled by the size. Turns out it’s the first genus I dismissed based on size, and it’s just a baby.
Here’s one more look at this well camouflaged species.