Figured it out. Create and/or modify an existing module to be a UI module. Create a "resources" folder in the public folder of the module and place the image there. You can then access the image from a Sass class inside the module's themesource directory using the following: url(resources/{file/image name})
FYI - found this poaching from the Atlas Core module
https://medium.com/mendix/setting-up-your-modular-design-system-in-mendix-9-6-eed2d0063411
create a public folder in your module directory, and point to it (this is where we store brand logos, illustrations, etc)