Yes you can shrink your image to fit in all the elements. There are 2 ways to achieve this:
1. Create a Blank Canvas and choose the Facebook Cover preset from the dropdown. Then drag on your image from the Photo Basket at the bottom of the editor and resize it to fit.
2. Or, you can distort your image to fit the Facebook Cover by using the Basic Edits > Resize tool, unticking Keep Proportions and entering 851×315. This will however make your image look a little squashed.
Let us know if you have any other questions!