Hi bigduffey & welcome to WebmasterWorld!
Note that setting the image src will not change the size of your image. Since you didn't specify it in your <img...> tag, and you didn't specify an image, perhaps it is 0x0, so when you change the src you still don't see it. If all you images are the same size, try set the height and width properties in the <img...> tag, or show one of the pictures as default. (If all your images are not the same size, then once you get this working, you will need to add more code to adjust the height or width, else you images will distort)
Note that you don't need the "new image", you can just do:
.....imagemain.src="images_europe/" + "randoMe_big" + ".jpg";
<Added: Oops, scratch that. You don't need to set the size>
[edited by: ShawnR at 10:16 am (utc) on May 11, 2003]