As I say in the help, you have to reduce the width of the browser window. There's a maximum image width (and height, tho that's less likely to be a problem given the normal landscape-style monitor) of 1500 pixels. The program sets up a buffer, which is normally 50% more, so you can pan the map a bit in any direction without having to load a new image. If you have a standard screen size of 1024x768 that will be just over the limit at full-screen, so I reduced the buffer size to 40% more. Of course, that won't help if you're using a higher-resolution screen, say 1280 - even 10% then would be too much.
If you look at the url of the image, you'll see width and height at the end; if either of those is > 1500, it won't work.
I'll look into making this more foolproof, giving an error msg before requesting the map image.