It depends really. Most of the photos I find online have very little information in the caption that leaves people questioning it. At times a do take the caption but add upon it with information I find during the research of photos (ei: the Queen Elizabeth post). Sometimes I already know the information I write in captions (majority of Beatles photos). I agree, it’s completely fair to source back someone’s words. But the majority of photos with captions I myself have come up with. If I do take someone’s captions, they’re (photos) most likely from books I’ve got and source back to where those photos were scanned from.
I hope that made sense, I tend to ramble on. Also sorry for a late reply, I usually check inbox every morning/afternoon/night.