Hi,

Please find attached a new version of the module, with some ideas
from Marco. Later, I'll upload it to modules.contextgarden.net.

Here a usage example:

\usemodule[downsample]
\setupexternalfigures[
  location=default,          % find hacker.jpg and mill.png
  cache=/tmp,                % where to put the low resolution images
  resolution=50,             % resolution in DPI
  conversion=downsample.pdf] % use this module
\starttext
\externalfigure[hacker][width=0.4\textwidth]
\setupexternalfigures[resolution=20]
\externalfigure[mill][width=4in, height=3in]
\setupexternalfigures[resolution=200]
\externalfigure[mill][width=4in, height=3in]
\stoptext

Hans, it would be nice, to apply this patch, to trigger new conversion,
when figure dimensions change:

--- grph-inc.lua~       2019-02-14 17:04:12.000000000 +0100
+++ grph-inc.lua        2019-02-17 08:45:38.129971581 +0100
@@ -864,6 +864,14 @@
                     newbase = prefix .. newbase
                 end
                 local hash = ""
+                local width  = figures.current().request.width
+                local height = figures.current().request.height
+                if width then
+                    hash = hash .. "[w:" .. width .. "]"
+                end
+                if height then
+                    hash = hash .. "[h:" .. height .. "]"
+                end
                 if resolution then
                     hash = hash .. "[r:" .. resolution .. "]"
                 end

TIA,
-- 
           Peter