To make the platforms for your game search "platform sprite" or add a term in front like "graveyard platform sprite" or "cave platform sprite". Put the sprite through the background remover.
Upload it as a sprite in your game and hide it. Use the select tool and draw a box around the sprite you want to add toy your game. CTRL+C to copy it, switch to your ground sprite, and CTRL+V to paste it. You need to replace the Ground and Platforms 1-3 costumes.
The costume tab should look like this when you are done.