It is my understanding that we are to ask permission from the owner of the image on FindAGrave, or any other website, but no one stops folks from uploading them daily. I've never been refused use of a photo, by anyone I asked permission from.
From our Help Index on use of photos:
How do you know if you have permission to upload a photo?
Generally speaking, you have permission to upload a photo if:
You generally do not have permission if you downloaded it from another website.
Commercial sites such as Ancestry.com and Find-A-Grave do not generally allow republishing images. Even non-commercial sites may disallow public sharing of downloaded images. You need to check their terms of use. For example, here is the Wisconsin Vital records terms of use.