For anyone else interested, here’s how it’s done:
1. Create a BLANK CANVAS with a white background.
2. Add text, shapes, or anything else you want to be filled with your image. Important: everything you add needs to be black.
3. Flatten your image by clicking the Cog icon up the top right next to Undo/Redo and then Flatten Everything.
4. Drag on your image from the Photo Basket at the bottom of the editor.
5. Change the Advanced Blend Mode on the Photo Properties dialog which appears to Lighten, and resize the image so that it looks correct.