How big is the layout / would capturing it on an image a max 2048x work 1:1 for example, or will you have to accept losing some resolution to cover the width/height of the laout?
No matter, I would suggest trying to use either the Canvas plugin (which lets you paste layers) or the Paster plugin (to which you can paste families and it's faster than Canvas). You can download either as an imageURL using the Browser invoke download action. I couldn't get Canvas to work with umbra just now (don't know why), so Paster seems like a better option IMO, even if you'll need to code a few lines of object pasting to get the image you're after.