Gravestones, particularly, as you mention, an image, not a transcription, is a good source. Just be sure that other information that's consistent with it doesn't, itself, derive from that gravestone! ;-)
Consistency is, in general, a good indication, Charlotte, provided that consistency is among multiple sources or references that are independent of one another. In many cases, such as ancestral files, trees on ancestry.com, Millennium File(s), family data collections, etc., the dates and relationships may well be all from one original source.
That is, one person may guess that John Doe is the son of Joe Doe, or John Doe was born on such-and-such a date. Someone else copies that info, which gets copied by yet someone else. After that information propagates, it appears everyone agrees that the original guess, now lost in time and space, looks like consistent agreement.
On the other hand, consider a birth record, a marriage record and a death record that all indicate John Doe was the son of Joe Doe (or born on such-and-such a date). Because the information in those records likely came from different people at different times, that consistency is more likely closer to the "truth."
[ninja'd by Jillaine]