I'm using a laptop PC, so this may work differently if you are using a phone or some other device; when I go to that article on minerals for the soil (or any other article) and hover my cursor over the photo, a red box with the Pinterest symbol shows up in the upper left-hand corner of the photo; clicking on that pins it to my Pinterest page. I don't know if there is any way to pin forum posts (which I realize wasn't the original question, just throwing that out there), but it does work for pinning photos in the database and in forum posts.
