Although this problem kept me busy for half a day I’ve now solved it by myself.
So the sidebar and the content were totally separated … one at the left, the other one at the right … because the wrapper was on 100% (and something else … just can’t figure out what it was). When I changed the wrapper to 750px I couldn’t see my wrapper image anymore. But when I added the same URL to the background, the image appeared again and it works all fine now when I resize the browser window. Perfect, I’m happy!