@rain: I think I pinned it down (and Picnik is probably innocent).
Like Monotone, selecting size option, changing width, etc., have no effect. The theme bg is confined to 840px wide, so above that you get downsizing and no color along the sides. If the image is less than but close to 840, you only get two very narrow strips of course, so my original 800px suggestion is a good limit. So what's the problem? Some smart guy set it so that you don't get the sides if the width or the height of the uploadeed file is above 840!
For the sake of others who might read this, I'm also reminding that for a centered image you need to set the alignment to None before inserting.