For complete creative control, I recommend using the blank canvas option. You know you're using a blank canvas when you have no dotted lines. Using a blank canvas means you are not governed by set spaces to place your images.
With a blank canvas you can:
- Layer and arrange images
- Use the drop-shadow tool to add depth and perspective
- Remove the background of an image
- And many more effects...
Here's a few board examples using a blank canvas: