why javascript fails-external .js file?

So, this script I’m using on my homepage
(http://danmoriarty.com), to rotate .jpg images,
that only seems to work when it’s embedded
in that page’s code.

When I try to add it to my external .js file, with
my other javascript code, it fails to work.
Anyone know why?

Here’s the code:

also:

and:

I’m trying to learn javascript. Thanks much.