Cees,
image files are raster, DXF is an object ('vector') format. You therefore
need to run some kind of raster to vector conversion (a mathematically pretty
complex thing). There are various ways to do this.
CADDVIEW (shareware in GO ARETAIL) allows *you* to trace the image and create
an DXF file.
HiJack Graphics (and similar programs) provide some basic automatic tracing
of images.
And then there are the real raster-to-vector programs that cost thousands of
dollars.
Dietmar Rudolph
Author, "DXF Standard" book