1) Create your collage
2) Click the "Done" icon on the Editing page
3) Click "•••" More
4) Scroll until you see "Save to Dropbox"
Note: You must have Dropbox on your phone.
Please refer to this collage:
If you want to add photos from Dropbox directly, sorry that we don't support that for now. But you can download your photos from Dropbox, and then add photos from camera roll to your collages.