Some templates (like the default J3 ones, and yours - apparently) set something like "max-width: 100%" globally on all IMG tags, and that distorts the tile display... i put a CSS override for that in the HTML header, which seems to work
this is most certainly a CSS issue. The OSM maps are made out of several tiles that get pieced together as IMG tags. If there's a CSS style that sets a fixed width or height on IMG tags, you can get distorted displays like that. If you have Firefox with Firebug, or Chrome with Developer Tools enabled, you can see which CSS stylings influence the IMG display.
I can try to take a closer look when i get back from Morocco this weekend.