Thanks for reaching out to us!
To achieve this:
1. When creating your Shape Collage, ensure “Hide Background” is ticked
2. Save your collage to your computer
3. Open up a new image that will be your background, or create a “Blank Canvas” with the background color you want
4. Re-upload your saved Shape Collage to the Photo Basket (at the bottom of the editor)
5. Drag it onto your main image and resize it appropriately
Let us know if you have any other questions!