If you add the following html to a NetWebPage, then the screen width, and screen height of the remote _screen_ will be sent to the server, and will be stored in Session variables for that user.
<script>
SetSessionValue('screenheight',screen.height);
SetSessionValue('screenwidth',screen.width);
</script>
Note that JavaScript is case sensitive, so you need to use the right case for (the JavaScript function) SetSessionValue.
In the templates, wherever you set an image, you also can set the height, and width, of the image. Use the sessionValues in a calculation there to determine how big you want the image to be displayed as.
Note this is the _screen_ width and height, not the _browser_ width and height. However for targeting mobile devices they are probably the same thing.
Cheers
Bruce