an external JS function to update a browser window other than that which invoked it

[Salam]

Hi, I know how to make an external JavaScript function update a field on a form on the browser window that invoked the function. What I could not figure out is how to update a field on a form on a browser window other than that which invoked the external JavaScript function.

Example:

function image(imageID)
{
// question is: what is the correct statment to use here ??
document.forms[0].image.value = imageID;
}

The window to be updated is diffrent than the window I select the image from.

Thanks,
Salam