@thisisasignin: after the error message "Could not find kernel image: linux" in my case appeared a prompt "boot:".

After this prompt I could enter a kernel file name and options. in my case worked:

ubnkern initrd=/ubninit file=/cdrom/preseed/ubuntu.seed boot=casper

ubnkern is the kernel filename relative to the main directory of my usb stick. the same is true for ubninit, the filename of the inital ramdisk filesystem image.

The other options I got from reading syslinux.cfg in the same directory.