I recently decide to add some pictures on the folders in Tableau Server, in order to improve the user's experience and put some joy in the data
it's quite simple, so no need to code anything with the new tableau server funcionality.
The first step is to collect some pictures, I did it going to Google/ images and after clicking on the image, right click : copy the image location
The second step is to go to tableau server/ and click on the folder/ details
click on add description
and just add between two ! the image location = ! image location!
ex : !http://www.image1.jpg!
and that's it! My users were happy and so do I.