I'm using Windows 10.
I'm in Wahoo Beach, in the Jungle Farm Plot.
1. Click on the tool to change your house appearance. Click on "Paint", click on "My Houses".
2. Select your house.
3. Drag any of the wallpapers of your inventory into any of the slots of the house.
4. Click on "Apply".
5. Repeat steps #1 and #2.
6. Drag any of the wallpapers from the slots of the house to your inventory.
7. DON'T APPLY, Click on "Cancel".
8. You should have the wallpapers you dragged before in your inventory.
9. Repeat steps #1 and #2.
10. As you didn't apply the changes before, you should find the wallpapers still in the slots.
I believe I could become billionaire if I keep doing this and sell all the wallpapers :P