Take a look at the result here:
http://www.hartdesign.nl/index.php?page=schilderijen
All suggestions more than welcome

Fixed. Turned out to be the good ol ownership issue. Recreated the dir in ftp and chmodded it againhmm that's weird.
thumbs were created for images inb the following folders:
http://www.hartdesign.nl/uploads/images/
http://www.hartdesign.nl/uploads/images/schilderij/
but not for:
http://www.hartdesign.nl/uploads/images/schilderij klein/
http://www.hartdesign.nl/uploads/images/schild/
thought it was the space in /schilderij klein/
but apparently not..............any idea Mark?