If you'd like to use a custom image, you have a couple of options.
Option 1: Choose from pre-designed images
Just open the Object settings. Click the Settings tab. Then, you'll see a list of different images at the bottom:
Option 2: Upload a custom image
Here's how to do that...
Step 1: Click on the Object and then click the "Customize" tab...
Step 2: Click the "Upload snapshot" link
Step 3: Browse to find the new image on your computer
Step 4: Preview the image, click the checkbox "Use snapshot on Canvas". Click Save
Step 5: Awww YEAH!