What a weird way to give people more space
Next, users must utilize Dropbox's new automatic photo import feature to upload photos (see notes below). This beta feature is actually the reason for the promotion, as the Dropbox team is working to stamp out the bugs associated with it.
On your first automatic photo import -- any amount of photos will do -- Dropbox will give you an additional 500MB of free storage space. For every 500MB of photos you upload afterward, Dropbox will add an additional 500MB to your account for a total maximum of 5GB of added storage space. This amount is added atop the standard 2GB that free users already enjoy, so keep uploading photos and you'll have a healthy 7GB of free Dropbox storage.
So the only way to get the 5 GB of space is to upload 5 GB of pictures and or video, which you can then delete and have the space free.
Guess that's one way to get people to upload photos.