Well thank you when someone compliments your FB photos is more than acceptable, but if your looking for alternatives
1. Give a one sentence background story to the pic
2. Say, something like "aw, you're very sweet"
3. Or if it was a day out with family/friends. "It was a great day, got some great pics"
4. If its got a connection between you, the complimenter and what's in the pic, say what happened and maybe tag the connection in your comment
Just be a bit more imaginative, and sometimes you don't have to say anything at all, just "like" the compliment!