pronouns - Another "and I" vs "and me" question

I see this a lot on Facebook. Two people doing a thing, and the picture captioned with:
"John and I waiting on the bus"
"Mary and I at the Beyonce show"
"Sally and I with our awesome guide Sven"

I think it should be "Joe and me" but I'm not sure why. Is there an implied "this is" at the beginning of each sentence, making Joe and me the object rather than the subject?