Mel, you need to change the size to around 800 x 600 pixels. If you've got Photoshop it's just open the image, go in the menu to Image, Size and then type in those values but remember to save it with a different name or you'll never get the quality back.
If you haven't got Photoshop then right click on the file and choose edit, see what program comes up and let us know.
Or email it to me and I'll do it for you
(click on my name when you're logged in to find my address)