Set the property displayImage of the INObject inside the INObjectCollection.

In the intentdefinition file you must set the parameter to Dynamic (check the Options are provided dynamically). Then you must have an intent extension that return the custom values (inside the provideXXXXOptionsCollection) (see https://developer.apple.com/documentation/widgetkit/making-a-configurable-widget for doc about custom widgets). You must generate an array of INObject with the possible value for the parameter. To set an image for a item you must set the property displayImage of the INObject.