I've bought one saddle online and was very satisfied with what I got, but like SR said, you need to know what seat size you want, the tree type (roper, barrel, trail, etc), and what size/angle the tree needs to be to fit your horse.
Buying from a picture only, I stick with bigger name brands that are known for their quality. That way, you won't be unpleasantly surprised when you open the box for the first time.
This was the saddle that I ordered and I couldn't have asked for better quality.
Navajo Roper Saddle by Simco - Simco