Cross Browser "Form Auto Submit On Enter"

I need a piece of javascript that will allow a form to be submitted
when Enter is pressed for IE6 and 7, Firefox 2, and recent versions of
Safari. It is for a login page that has two textboxes: a "Login" and
a "Password". The "Login Button" is actually a standard image with an
onClick event handler to submit the form, because we use a mouseover
effect. (That is, it is not INPUT TYPE="IMAGE" but rather an IMAGE
tag with added.)

I found some code that works in IE, which is below, but it only works
in IE. I am sure there is a simple cross-browser fragment of
Javascript that will do the trick, so please point me in the right
direction. Code: