Now I can downsample jpeg-files on the fly, see attached example file.
But I need 2 patches:

1.) To get the width and the height into the name of the file,
    because when the figure dimensions change, there must be a new
    conversion:

--8<---------------cut here---------------start------------->8---
--- grph-inc.lua~	2012-06-05 10:37:01.000000000 +0200
+++ grph-inc.lua	2012-07-31 22:40:39.214426838 +0200
@@ -528,6 +528,9 @@
                     if resolution and resolution ~= "" then -- the order might change
                         newbase = newbase .. "_" .. resolution
                     end
+                    local width = figures.current().request.width
+                    local height = figures.current().request.height
+                    newbase = newbase .. "_" .. width .. "_" .. height
                     --
                     -- see *, we had:
                     --
--8<---------------cut here---------------end--------------->8---


2.) To avoid crap in the width and height options:

--8<---------------cut here---------------start------------->8---
--- grph-inc.mkiv~	2012-07-20 23:26:52.000000000 +0200
+++ grph-inc.mkiv	2012-08-01 00:02:23.851094854 +0200
@@ -293,6 +293,10 @@
    %
    \the\t_grph_include_local_settings
    \dostarttagged\t!image\empty
+   \edef\current_width{\externalfigureparameter\c!width}%
+   \edef\current_height{\externalfigureparameter\c!height}%
+   \def\nocrap##1{\doifnotemptyvalue{##1}{%
+       \the\dimexpr\csname##1\endcsname\relax}}%
    \ctxlua{figures.push {
         name       = "\p_grph_include_name",
         label      = "\p_grph_include_label",
@@ -312,8 +316,8 @@
         resolution = "\externalfigureparameter\c!resolution",
         color      = "\internalspotcolorparent{\externalfigureparameter\c!color}", % hack is needed
         ["repeat"] = "\externalfigureparameter\c!repeat",
-        width      = "\externalfigureparameter\c!width",  % can be crap
-        height     = "\externalfigureparameter\c!height", % can be crap
+        width      = "\nocrap{current_width}",  % no more crap
+        height     = "\nocrap{current_height}", % no more crap
     } }%
    \ctxlua{figures.identify()}%
    % also mode: checkpresense only
--8<---------------cut here---------------end--------------->8---


Hans, could you please patch the files?

Cheers,
-- 
           Peter