Within C5's php, block controllers can have a delete method, so it may be considerably simpler to write a delete method for the block that does most or all of what you want.

If you still want to do it all with javascript, in edit mode, deleting a block is an ajax transaction between the browser and the server, so it should be possible to adapt the javascript involved to trigger a javascript event that you could then listen for and remove the unwanted image. You could also simulate such an event with a polling loop. You would then need to use javascript to simulate the C5 process for removing the image with further ajax transactions.