To add custom shapes, create a folder in the following location;
C:\Users\YourNAME\Documents\My Tableau Repository\Shapes
Then in Tableau open the Shapes menu and click refresh. The folder you created should appear.
I use the blank shape often. I create a blank image in software such as Tech Smith's Snag It. You can hide certain shapes with the blank option. I have a use case in my tableau dashboard here. In this example, I don't show the enemy icon until all the material in a level is complete.
Luke Stanke lists more use cases for the Blank shape icon here.
Mouse Arrow or Hand Icons
I also like to have a Mouse arrow in my dashboard as a button image to signify to users to open a floating container. I have the Hand icon when the container is visible. The example dashboard containing these is here.
I use the filter icon to hide my filters in a floating container. This frees up space on my dashboard. Sample dashboard is here.