a) Yes, you just replace the two zeros with the bit I suggested:
background: url("http://imveryape.files.wordpress.com/2012/02/justice_in_concert-60.jpg") no-repeat scroll 0 0 / 100% auto transparent;
(Or with a color code instead of the word transparent - see end of #c below).
b) I know how to do a media query, but I don't know where I should set the limit (or what other options might be better) because I don't have direct experience with mobile devices. So you probably have to wait for a staff reply on this.
c) Your image has reasonable proportions, but screens vary in proportions, so there's no way to ensure that a non-repeated image won't leave any blank space no matter what the screen (except if you force 100% width plus 100% height - but this would make the image ok in some screens and distorted in others). That's why it's better not to set the bg image the way you've set it: the best solution would be position top center instead of top left, with an image that fades to a uniform color (black or grey in your case) left, right and bottom, and the same color set as a background color to the blog (so that it's seen as a continuation of the image instead of blank space).