Creating Custom Backgrounds on Inkarnate [WORKAROUND]
Inkarnate doesn't have an "upload image as background" feature by default. The known workaround is to upload your image as a texture, then scale and off-set the image until it's about the size of the canvas you selected, then fill the canvas using the menu. It's kind of obtuse. Linked below is a youtube video going over the process, though the UI elements are out of date, so you'll have to figure that out on your own.