This Applet is terrific. It will create a folder in Dropbox named "Send to iPhone" and for every photo you add to it, the Applet will automatically place it in your iPhone's Photo Album named "From Dropbox."
If
New photo in your folder
This Trigger fires every time a photo is saved in the folder you specify. NOTE: will process up to 15 photos per polling period.
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Discover more time saving integrations for iOS Photos and Dropbox