The Form runtime template needs to show the image that the form builder has selected. Assume that a property url is set (see later on). Note the use of ng-src (see AngularJs docs on ng-src) to have a dynamic image:
Note the syntax field.params.customProperties to get access to the non-default properties of the form field.
The Form editor template simply needs to be a generic depiction of an image or even simpler like here, just a bit of text
<i>The custom image here</i>
Don’t forget to add a property url to this stencil item with the name url and type text.