Firstly, most people know exactly as much as you do (or less) and that's why a lot of the stuff on the workshop is pretty poorly made with limbs out of place and such.

Unfortunately, there's no way to upload custom bitmaps (although that would be beyond amazing), so you have to make do with the in-game items and body parts. However, Scribblenauts has an enormous collection of those things, so don't underestimate the game's capacity to have a stamp that will (almost) perfectly fit your requirement.

There's not really any tutorials for making characters using the editor, although HenryRichard has an excellent guide on making avatars and the principles of that guide also apply to making characters with the object editor. Here's his guide, give it a read:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=154743015

Since I have been making characters for a few years now and have everything I need to record a video, I have been considering creating a video tutorial on creating characters for Scribblenauts Unlimited. Based on your post it sounds like it would likely be well-received. What do you think?

For now, you can just friend me and I can give you tips and pointers on improving your characters :)

Cool, I have been out of scribblenauts for a while, but I would definitely like to see a video of how one of those really cool characters is made.