Other people's pictures showing up on phone

Question

Hi,
I have a bit of an usual mystery to solve. A few weeks ago my mom noticed pictures of strangers showing up in her gallery. Oddly, they appear to have been taken in a foreign country, as one picture shows a road sign in what appears to be Russian (?).  There was a grand total of 63 of them. According to the gallery, 1 pic was taken on July 27th, and the rest on August 5th. The pictures are stored in what appears to be the normal picture directory for the phone (My Files > Internal Storage > DCIM > .thumbnails), alongside the pics she has taken herself. My original hunch is that some sort of photo sharing app is to blame. She first noticed these pictures during a family vacation. We were in a touristy area, and so it is possible we walked past vacationers from different countries who happened to have some sort of wireless photo sharing app enabled on their phone. However, looking over the apps on my moms phone, there are no obvious photo sharers. One thing that does look suspicious is the "Media Share" setting inside of the Gallery app. However, it was not turned on when I went into it (and I assume, by default, is turned off). I highly doubt my mom is capable of turning it on herself (she is not tech saavy). Furthermore, even if it was turned on by her accidentally, it's set to only connect to trusted devices, and will only work over Wifi (To my knowledge, her phone never connected to a public wifi network while we were vacationing - which is also strange, wouldn't most apps wait to transfer 63 pictures over wifi to conserve cellular data?). And actually, to my knowledge her phone has never connected to a wifi network at all, not even our home network because she uses it so little. Any ideas as to what caused this?
A few other bits of info:
-My mom has not set up any social networking accounts, so it is not something synching from there
-She has not opened any strange emails with attachments
-Her downloaded apps consist of running apps (eg MapMyRun), coupon apps (groupon), and popular apps like pandora and hotmail. Nothing that is explicitly related to photos or media.
-She has not noticed anything else that is out of the ordinary on her phone

Answer 1

The phone was purchased from our local Verizon "Premium Retailer". It was sold to her as a new phone, and I have no reason to believe that the salesperson would deceive her with a used one. By the way, we did have the folks at that store take a look at it the other day. They were very nice, but were equally puzzled, and couldn't come to a resolution. The memory card in the phone is the original, it is not new. To my knowledge, and to hers, the card has never been removed (and there is really no reason for it to have been removed in the first place). I know that the pictures can be removed, which is good, but I'm interested in figuring out how this actually happened, and if it could happen again. Is there any way to tell where a picture file came from? I know that pictures sometimes contain location data, or information about the camera used, but is there anything else that could be a hint?