When naming objects on a page, most commonly images and form elements, NEVER name any object with the same name.
For instance, let's imagine you had a rather simple page like this:
document.USER_INFO.USER_ID is undefined
The same would be true if you were trying to access an image object with the same name as a FORM element that it followed. It can't find the image because it occurs right after a FORM element with the same name. Since .src is not valid for a FORM element, it returns an error.