Add these two lines:

void drawImage(in ImageData image, in double dx, in double dy, in optional double dw, in double dh);
void drawImage(in ImageData image, in double sx, in double sy, in double sw, in double sh, in double dx, in double dy, in double dw, in double dh);

to:
http://www.w3.org/TR/2dcontext/#conformance-requirements

Usage scenario. You have a large image that you want to render a portion to the screen without overwriting the whole canvas. (Pretend part of it is transparent). You also want the ability to change pixel info quickly and ImageData allows that.