Check your editor's extensions in plugin.xml file. Ones you open up the
plugin.xml file and switch to "Extensions" tab, look for
"org.eclipse.gmf.runtime.emf.type.core.elementTypes" extension. Open it up
and you shall see all element types that you have in your editor. Now if
you select extension for any of the types, you should be able to set the
path to the icon image file from the extension details (on the right hand
side from the list of extensions look for "icon" text field with the
"browse" button).
Hope this helps.