Hi Liz,
Errors can happen anywhere, anytime, and some things, like names or spelling of names can change over the course of a person's lifetime. We all have seen gravestone, census, and birth or deaath certificate errors. "To err is human; to really foul it up takes a computer!"
I consider a photograph of a any data evidence created at or near the time of an event (i.e. birth certificate, Bible rcord, marriage license, census data, gravemarker, etc.) as a primary source. Everything else, I consider secondary resources. The source or resource just tells the reader where the data was found. There will sometimes be conflicting photographed sources, and those should be noted.
The important thing is to add whatever sources or resources you can find to as many WikiTree profiles as you are able. Links to on-line sources and resources are greatly appreciated! I always prefer to be able to look as the source or resource myself, but I can't get to a good genealogy library very often. Links are crucial to our WikiTree development.