how do you add a thumbnail.

how

Thumbnails are taken manually by me whenever I get the time, I run a script on my computer that runs through newly uploaded level and screenshots them.

principia-web predates the source code release so I originally did not have access to the official automated screenshotting tool. Now I do, but I haven't gotten around to implementing it yet.

if i remember correctly i remember building a thumbnail-like area outside of my game maps, then i use a camera (which is in game) for the thumbnail as it is uploaded onto the og site.

i needed no fancy scripts to do all the job, it just takes a little building skill and camera placement (for me) to finish what should be done

The way principia-web level screenshots are taken is very different from how they were taken on the official community site (they don't respect cam markers, are taken at the last save position, and are taken by a script I run manually on new level uploads). See the FAQ for more info.