Remco,
I remember there being a slight issue on some machines where once you set the default image dynamically, you have to press the tab or enter key while inside that input field. Try that to see if that fixes your issue... also be sure you have the latest build of DFP2... they released a beta version that will be listed in your account that should fix the problem as the first version of DFP2 had a problem setting the value for default file.
Cheers,
Brian