1. Go to your file on the dropbox website
2. Open file and click on the blue 'Share' button in the top right corner of the page
3. Click on 'Create a link'
4. Click on copy link
5. This is what that links looks like
6. Change the link so that https://www.dropbox.com is replaced with https://dl.dropboxusercontent.com so in this case it would look like this
7. Put this link into the image URL column in the spreadsheet
Note: You may also bulk edit the URLs directly on the template file by using Ctrl+F and replace Dropbox image links