OK, I figured it out. I was able to center the radar image within the observations box. But because the box had a different aspect ratio than the radar image (wider than the radar image), I had "pillar boxes" along the sides that were still the original blue background color.
So I took pan_texte.png (the file for the box) into Photoshop and changed the inside color to black. Then I changed the name of the file and the line 446 in styles.css to reference the new file, and it all seemed to work.
-Tom