I couldn’t find any tutorials or guides on this so I thought I would save everyone the trouble I went through figuring it out.
You can upload UI images at: https://create.crayta.com/images/uploadui
After you upload an image it gives you a url: https://live.content.crayta.com/ui_image/{IMAGE ID}_ui_image
After the image is approved, you can load your image in a browser window using the url.
But how do you actually access it in a game?
You can use it like you normally would use an image in HTML and CSS, just make sure you use the entire url, including the “https://”.
I tried loading arbitrary images off the internet and none of them loaded, as expected.
On another note:
I am concerned by the 6 image maximum. On the image management page(https://create.crayta.com/images/mine_ui), it says “Once you set an image as a game cover or supporting image, you can safetly delete it from this page.” This is confusing because the images uploaded there are for UI and not game covers or supporting images. I’m assuming deleting the these images would make them no longer available for UI. This means that across all my games and packages I can only use 6 images. And if I publish a package with custom UI images, deleting one of the UI images would break the package for anyone using it in their game.