We do repairs on a large number of phones running all the phone systems, iOS, Android, Windows and Blackberry, almost all have pictures on, I don't think we have found quite the ratio as these researchers though. It's almost tempting to set up a tumblr account and rake in the advertising money. For those asking, the best way to wipe your phone before trading it in, selling it on, disposing of it is a factory reset;
Android has this in the settings menu, under privacy for some older models and under settings, back up and reset on newer kit
Windows phone is under settings and about phone
iOS is under settings, general, reset, erase all content and settings
Blackberry could be anywhere, it seems to change all the time, best to RTFM and be prepared to wait, it can take up to an hour for some to wipe and reset.
It's best to go through the new phone setup procedure at least once, to check everything has gone and then reset one more time. If your phone has been modded from its stock ROM be aware that these steps do not always work and your photos will stay until you manually delete them all.
Hope this helps.