(1) Go to the PennyArcade website an see what their requirements are with regard to posting their work. Follow their requirements.
(2) You have chosen one of the narrow themes. In fact it may be the most narrow them there is. If you swtiched to this theme it is widerand will cover the whole screen WordPress Classic 1.5 by Dave Shea.
(3) But I don't think you're really getting the point here. The point is not to post huge images it is to post thumbnails like lorelle said: "Or make it thumbnail so people have to click it to see the full image. That's the easiest solution." In that case you get a photobucket or flickr account. The images are hosted on the 3rd party server and not on wordpress servers.
(4) No there is no way to download other themes without purchasing the upgrade. The other wordpress thmes (over 800 of them) are for wordpress.org and are on a different code base than we are on. We are on a multi user base and all of the themes must be re-written to be used on wordpress.com.