I know you really want that!
I was doing it but then realised the "normal bug" in the forums which led to disabling using external images in the forums.

The following part is an explanation of how it would've worked:

- You put an image in your signature
- You reply to a topic
- The system detects new replies
- It sends you a push notification

Of course this only works if the reply is in the same page as yours. But you get it, how could would it have been?