Setting an image from an external ASP page

I don't know if this is possible, but here's what I am trying to do:

I have a javascript on a ASP page which references another ASP page to set an image on the first ASP page.

This is the javascript on the first ASP page which is executed from a button click:
My vbscript/ASP question is:
The ImageBuilder.asp file uses vbscript to select the correct image from the parameters passed via 'Fields'.
What code/syntax do I need to use to return the correct image path to the first ASP page?

I've seen this done in PHP (using ECHO), but I am hoping it can be done in ASP.