Gregory,
As Jkelly has explained, we don't need a secondary source claiming that a picture of a tree is a picture of tree before we can insert it in an article.
Quoting Gregory Maxwell gmaxwell@gmail.com:
and we would likely not need a secondary source to place this image on an article about protest posters.
I think that there is something smart about making this distinction, which I failed to do, but I'm not sure that it is one we usually make. If I state that my tree picture shows damage from acid rain, do I need a source for that? What level of interpretation are we comfortable with?
Jkelly