You can use the Snapshot action instead. For for a 100x100 image centered on position 50x50 you could do this:
---Set canvas size to 100x100
---scroll to (50, 50)
---set canvas size to 640x480
---scroll to (320,240)
---invoke download of canvasScreenshot
The second scroll to is to scroll back to the original scroll location. If the layout isn't using unbounded scrolling you could also use (0,0).