First of all, thanks for your help PeejAvery,

I did it like you suggest, but I'm experiencing some problems.

this is the line of code for changing the picture:

document.getElementById('ctl00_ContentPlaceHolder1_lblimage').innerHTML = "<img id='userimg' width='200' height='240' src='" + document.getElementById('ctl00_ContentPlaceHolder1_FileUpload1').value + "'>";

Its just the same way you suggested, just with some long id's.
On the first view it seems to work, but IE7 does not show the image after all!

I made two screenshots for demonstration, this is the above line of code outputed with alert:

http://meet2poker.tempw7.internet1.de/temp/output.jpg

As you can see, it executes the code correctly, but the image is not displayed.

Second screenshot shows the information about the picture:

http://meet2poker.tempw7.internet1.de/temp/file.jpg

It looks like.. he still does not know where the file is located, it just got the filename without the path ?!? I don't know why.. but the src line is correct as you can see in screenshot one.

Doohh I'm confused.