The symbol entry depends on the OS. You can swap fonts to use the Symbol font or you can use the OS method of creating unicode characters.
To show a static picture, or a limited set of pictures, there is a control called the Picture Ring. Import the pictures into the named/numbered set, and set the value to whatever you like.
For more extensive picture control, you can look at using the 2D or even the 3D picture control. These get more complex as they are actually more like canvas controls with complete drawing APIs on the diagram.