Computer uploaded old photos into iphone6s, how do I delete?

[iMore Question]

I use iTunes to upload my photos from my iphone6s to my windows pc. Well last time I plugged it in everything seemed the same except it somehow added about 200 old photos from my computer back onto my phone, but I can't delete them using my phone gallery. And I can't figure out how to do it thru my computer either. It's taking up a lot of memory. Please help.