Here is a great article covering this topic - Put some Life : How to add pictures on a folder in Tableau Server
I hope this helps
Please mark this reply correct if it resolved the question so others may easily locate the same response.
My problem is the same except I could find the folder on server. When I placed my images there and used the html in the project description as per the article (which is where I started) I get a broken image thumbnail like the original poster.
Is there a permissions/enabling thing in server that will make this work?