Harald,

I use ImageMagick/GraphicsMagick to resample HUGE high resolution tiff
images to something more manageable to work with. Using a script, I
converted hundreds of images to lower resolution copies. Then it was
simply a question of pointing ConTeXt to use the appropriate directory
to find the right figures.

The basic comand is

convert -resample 300x300

(or 100x100) and you can play with "-quality 75%"

This can be done once and is much better than getting ConTeXt to
convert every time on the fly.