1) Create your collage
2) Click the save icon on the bottom right of the page
3) Click "•••" More
4) Scroll the bottom bar 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 add photos from camera roll to your collages.