It seems that you can share photos by accident...
At church, we quite often get large PowerPoints and video files for use in services. They used to get emailed round to a bunch of people, which was a bit silly. Now we have a Dropbox account with folders shared with the interested parties, and files are either dropped or emailed there. The Dropbox client is installed on the projection PC, and used to collect the files to set up the services.
The other week, I found that a large collection of images had appeared in the account. I could guess whose they might be from the content, but no-one had any knowledge of putting them there.
I've finally figured out that Dropbox, by default, offers to synchronise images from any USB sticks inserted into the PC. Someone must have used a stick to transfer one file to the PC, and accidentally added 5 years of holiday snaps for everyone to enjoy.
No harm done, but it doesn't seem to be made very clear to the user what's happening...