if you want a single category, switch any category to one you love
If
New Editors' Choice photo
This Trigger fires every time a new photo is added to the Editors' Choice page
Then
Add file from URL
This Action will download a file at a given URL and add it to Dropbox at the path you specify. NOTE: 30 MB file size limit.
Explore more great ways to automate 500px and Dropbox