So you can use any custom shape in Tableau. To add to the default list...
Get the icon you want Free vector icons - SVG, PSD, PNG, EPS & Icon Font - Thousands of free icons is a great place to get them (and you want them in .png so you get transparent background)
Create a new folder in the shape folder in My Repository (usually in My Documents)
and copy the icon/image into the new folder (you can see I have one for AdVizture, Atheon...etc)
You will now have access to this, when you select your shapes in Tableau
If you can't see them, hit the Re-load shapes...
Let me know if that doesn't make sense.