How does the caching in MySB image proxy work ?

I'm asking because I need to know how much bandwidth it will take from my web server when a lot of applet instances requests an image through MySB image proxy.

The scenario looks like this:
1. I'm going to have a few images which is about 4MB size each on my web server
2. The images are going to be refreshed every 30'th minute or so
3. I'm using apache on my web server and the images will have static urls
4. In an applet I'm going to retrieve the image through MySB image proxy about every 30'th minute if the applet (screen saver) is active.

There are of course going to be many instances of the applet, one per Squeezebox it's installed on.

Two questions:
Will MySB image proxy make sure the image is only downloaded once or just a few times from my server during a 30 minute interval as long as the image haven't changed ? Even though there are a lot of applet instances requesting it.

Will MySB be able to detect when the image has been updated and download a fresh copy the first time an applet requests it after it has been updated ?

A sample image is this one:

The reason it needs to be downloaded again every 30'th minute is that the daylight shadow and clouds are updated.