There are more specific steps to do this found in this thread: Loading image file in thingworx repository to image widget in mashup
Just replace the textbox/button combo with a list and instead of using the click event in the button, use the SelectedRowsChanged events of the list.
Hope this helps!
There is another way, you need to use value display widget on mashup and bind the service GetPropertyValues dynamically to the value display widget. for that you need to persist the images for particular names as a property .
whenever you click the name the related image will display.
Try it ,it will work .