upload images: no thumb-URL created

This is so stupid, I’m afraid I’m overlooking the obvious again?
When I use the upload option, upload a file, click ‘create thumbnail’ (fixed size or custom size) all works well (the thumb-img.jpg IS created on the server in the same imagefolder as the original larger image), then I get the screen that says “here is the code to display etc.”, but this code is always the code to the large image, while I want the thumbnail to be displayed!
I searched throught the php-files but I can’t find where to edit/hack the code for the ‘ fileupload_url’