Refresh ajax panel

I’m using an ajax panel to show an image hosted at local webserver. I’ve used the iframe example on bottom of the panel config.
The URL ist something like “http://localhost/forecast.png?reloadtime=10”. This is a static url, but the image itself is changed every 10 minutes by a python script (same image filename).
The javascript parameters contains only “interval: ctrl.template(’$__interval’)”.

My problem is that the panel doesn’t refresh the image. The image is shown static, but never refresh.
I’ve searched the web but didn’t find the right way to do the refresh every x minutes.

Many thanks for suggestions.