Figured out the workaround to add custom thumbnails to your worlds on Race The Sun Workshop, and I believe it's important that every one have a fair shot at sharing their creations and making them look the best they can.

1. create custom world
2. create custom 800x600 PNG image (other dimensions will distort)
3. navigate to the Steam files (on your local computer)
4. find your custom world PNG file
5. replace the automatic PNG file with your custom PNG file
6. share your world to Workshop

Now go make your worlds look the best you can! Check out DLV Fast Future for an example of all of this with extra screenshots and a YouTube video added.

TIP: Every time you share your world to Workshop (to update with changes), the title and description will automatically overwrite whatever is already in Workshop. I suggest you keep a copy of your world description (and thumbnail image) offline in case you need to copy/paste or reupload. Avoid frustrations and be prepared!