\startluacode

figures.converters["png"]["lowres.png"] = function(oldname,newname,resolution)
    os.execute(string.format('gm convert "%s" "%s"',oldname,newname))
end

-- figures.suffixes["lowres.png"] = figures.suffixes.png -- suffix is automatically done

\stopluacode

\starttext
    \externalfigure[mill.png][conversion=lowres.png]
\stoptext
