class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
more info about the class
As it functions with jpegs of bigger size (see above) I was sure that I don't run out of memory.
But I stored it again as jpeg and everything is fine now. So, it must have been something wrong with the jpeg :-)
Thanx again