That photos tab shows you photos from anywhere in your Dropbox account, so these photos may be from shared folders you're a member of. (Likewise, other people can only see your photos if you shared them with them.) Another thing that may happen is someone "camera uploaded" their photos to your account (e.g., by plugging into your computer and choosing to import photos when Dropbox asked.) If you're still not sure where they're coming from, you should open a ticket: https://www.dropbox.com/support/s/92/3201098/c/2