Not open source but freeware and really simple and powerful. Absolutely no BS program and it just gets right to the business.
I've used it for years on Windows. I wish there was a similar program for Linux thats just freaking works.
Forgot to mention that it is stupid farst since it preloads images into memory so it can display images instantly hence the name. Perfect for browsing network shares. I only used the faststone image viewer, so not sure how the other programs work but I'm sure they are great too.