Move image off screen [SOLVED]

Thank you Ganblejs!
I've now got a solution, it isn't quite as smooth but does at least work.
Create a temporary image, the size of the display, use drawImage to draw a section of the large image to that, then display the temperoary image with setLCDOverlay:

var temp_img = Graphics.createArrayBuffer(g.getWidth(), g.getHeight(), 8);
var y_pos = 0;

function onDrag(e) {
        y_pos += e.dy;
        temp_img.drawImage(img, 0, y_pos);
        Bangle.setLCDOverlay(temp_img, 0, 0);
}