Random Photos from my Other Phone Appearing on my Phone

[Twisted_Imp]

So me and my dad have the exact same phone, the LG Optimus and this morning he shows me his gallery and several photos from MY phone are on HIS phone. Any idea how this happened and how to prevent it? Also the images can't be deleted off of his phone :S, which is even more odd. Please help, this is a big, confusing, privacy issue. Thanks.