"can't get it to work" is a bit vague.
Exactly how does it fail for you? And a screen shot of your layout taken while in layout mode would show others more about how you have designed this layout.
Is the Container field on the LEFT formated as a Container field?
If it is, you have to select the field and then you can:
and it should work without issue.
You do not need a script. I have written a similar solution for a customer and I created a button, then wrote a script for them to keep it simple so they could insert an image into a Container field. Do you require that or is the pulldown menu command more than enough?