Can't be done. You could have the image stored on an HTTP server and use xfa.host.gotoURL("/images/foobar.png");
The attached form lets you toggle between two images. The form has the same image embedded as image_1x1 (1in x1in and set to scale proportionally) and image_2x2 (2in x 2in and set to scale proportionally). Since image objects are not exposed to the event model I laid buttons over each image with the same proportions and removed all visual properties (caption, border, background). So I have btn_1x1 and btn_2x2. Initially, btn_2x2 and image_2x2 are set to hidden with the result, below.
When you click on the image,above, you are clicking btn_1x1. I then hide image_1x1 and btn_1x1 and make image_2x2 and btn_2x2 visible.
When you click on the image,above, you are clicking btn_2x2. I then hide image_2x2 and btn_2x2 and make image_1x1 and btn_1x1 visible.