You can add a fully stretched background to your button by following these steps:
1. Select the Content tool in the gray navigation bar.
2. Select the Page where your button is.
3. Click on the Design this page in the sidebar on your right.
4. Select the title of your button in the Design panel on your right.
5. Scroll down until you see Container and next to Background select Custom.
6. Select the Pick From Library button and upload your background image.
7. Select Save
8. Select Publish