The photos you download from Dropbox appear in Google+, when you have installed Google+ on your Android phone and 'Auto Backup' feature is enabled. If you enabled Google+ 'Auto Backup' feature, it will upload any new photos from Camera Roll to Google+. So if you download photos from Dropbox, it saves in your Camera Roll and then upload them to Google+. To prevent an automatic photo upload to Google+, you need to disable "Auto Backup" from Google+ mobile app settings. To do so, Open Google+ app on your mobile phone and tap 'Menu' icon located at the top left on the screen. Select 'Settings' and tap 'Back up & sync'. Switch off the Back up & sync to disable it.