Click the Looks category.
Drag out a switch backdrop block. Snap it below the go to block, and choose 'next backdrop':
Now, make the backdrop change when the Bearstack reaches the edge of the Stage.
Here's how you can build that script:Build your script to look like this:
Tip: Make sure the go to block snaps just below the switch backdrop block.
Type the same numbers in the go to block as in the beginning of the script. This will tell the bears to go to the starting point after the backdrop switches.