What kind of backlinks are these?

My competitors have lots of backlinks from sources like this one pharmacyusa-online.com/

Everyday 10-20 new backlinks come from this kind of sites on competitors websites, I sent them an email but never replied, maybe they are automated sites selecting pictures in exchange links or something?

Yes. the website seems to image accumulator. so they link back to you competiors images.

And how do they work? automatically find images and add pictures with a link back to the source? I have a few of them but my competitors have thousands of them.