I tried to create an ISO image this way:

$ genisoimage  -b ../floppy.img -o dos_wdidle3_1.03.iso wdidle/
genisoimage: Uh oh, I cant find the boot image '../floppy.img' !

According to the manpage, the argument for the -b option has to be relative
to the source path, so ../floppy.img was correct.

What the manpage does not mention is that the image must be _inside_ the
source path.

So there are 2 issues:

1. the manpage uses "relative to" instead "inside"

2. the error message is pointless and misleading, because it suggests that
   the image can not be found because the file does not exist