You can add a fully stretched background to your button (or many other blocks as well) by following these steps.
1. Select Design in the Dashboard navigation bar.
2. Select Sitewide in the righthand design panel.
3. Select Styles, then select Content Blocks.
4. Tick on the box that says Stretch block background to container edges.
5. Select Save.
1. Select Content.
2. Select the Page where your button is.
3. Select 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 in the blue box and upload your background image:
7. Select Save.
8. Select Publish.