To find out if your photos/illustrations are being used on a web site do a reverse image search on Google by doing this...
Open a Google Search web page and click on "Images" in the upper horizontal menu. Then drag and drop your image onto the search box. This tells Google to go out onto the net and search for that image.
I've found about a dozen of my images being used on websites from various parts of the world. And a few illegal uses of my images also!
I've found the best results by using the medium sized preview image on the DT web page for that particular image. I've tried using the little preview photos (thumbnails) from a main search page, but there weren't any results.
Well buyers so contact you outside DT sometimes. Even when you don't ask them to. I have my Facebook page link there on my profile. I have had 2 people there. Just make sure you don't ask them to contact outside. If they want to, they would contact you via your website. I suppose that's what the website field in profile does. Contacting directly for offline purchases is not allowed.
I only sell through DT at the moment. So it's the same thing. Outside buyers contacting me would buy through here only. DT gets the revenue, whatever happens.
55-250mm standard lenses. Dual tube macro flash and external speedlit...