For 3D models, like condo items, player models and vehicles, there could be a default thumbnail generated by TU by taking a screenshot of the model itself. This would solve a lot of issues where workshop items have images that aren’t representative of the model, or are a screenshot of it in other modeling software, which doesn’t match how it looks in-game.
Gmod does this for dupes, for instance: Steam Workshop::Destructible Tower Fortress
You can still set your own thumbnail, but it provides a convenient default and makes the workflow of uploading items more convenient.