@Metasomatism: Your comment lead me to the solution. If you change that to an answer I'll gladly accept it.
The command that does the trick is
for /f "delims=?" %f in ('dir *.jpg /b') do convert -size 1024x768 xc:skyblue "%f" -blur 0x25 -geometry 1024x768 -gravity northwest -composite "%f" -geometry 1024x768 -blur 0x25 -gravity southeast -composite "%f" -geometry 1024x768 -gravity center -composite "new_%f"
The commandline for ImageMagick is essentially this:
convert -size 1024x768 xc:skyblue
in.jpg -geometry 1024x768 -blur 0x25 -gravity northwest -composite
in.jpg -geometry 1024x768 -blur 0x25 -gravity southeast -composite
in.jpg -geometry 1024x768 -gravity center -composite
which first creates an empy (well, light blue) canvas of the desired size (here 1024x768 as the images will be shown on an XGA beamer). Then the picture in.jpg is added three times: once in the topleft corner being heavily blurred, once more in the lower right hand corner, also blurred, and once in the center, this one unblurred. This yields the desired effect. It also handles panorama pictures, i.e. pics that are in wide landscape mode. Here, the blurred image is added to the top and bottom, whereas normal portrait pics the blurred pics are to the left and right.