I find the same thing - really I think it is just that a few posts get the large number of responses, for whatever reason, and the majority get under 20 responses. Who knows what it is about any one particular post that catches the imagination of the board. (shrug)
Just keep posting pictures, I love seeing everyone's work even if I don't have much to say about it sometimes. I probably respond to about 20% or less of the pics I look at.
Also make sure you come back to your thread and respond to others' questions/comments.