Do an easy Google Image search on a photo and see if it pops up on someone else's social media account or elsewhere online.
If they have only sent you one or two photos, it is likely that they took that photo from someone else's social media page or from somewhere else on the Internet.
Video chatting with a person that you meet online is a common, normal practice in online dating.
In fact, all the couples I know who met online said that their first real conversations were over the phone or by video chat.
You can also find legal documents like bankruptcy filings, divorce records and sometimes marriage licenses. When people are communicating online, they will frequently send each other photos in real time.
Some cities and states even post traffic tickets online. During a conversation, ask to see a photo of the person right then.