I'm sure someone else has a better idea, but I can think of two ways to check to see if someone has linked to you.
In your stats, if a page with photos seems to be getting a lot of hits directly from the outside rather than from a page within your site, that would be a good clue.
In Google, you can do an advanced search to find out what other sites have linked to your site.
For Natural Images and Monarch Mountain, you currently have only a few sites linked to your home page.
If you find a page in the stats that seems to be getting a lot of hits, try Google to see who has linked to it.
I don't know how this would work if they linked directly to the image rather than to the html page, though.
I am going to watch this discussion to see what else is suggested.
This is an interesting question.