% For Mari Voipio

\startluacode

    local files = { }

    function document.collectgraphics(pattern)
        files = table.merged(files,dir.glob(pattern))
        files = table.unique(files)
        table.sort(files)
        logs.report(string.format("graphics: %s",table.concat(files," ")))
    end

    function document.choosegraphic()
        if #files > 0 then
            context(table.remove(files, math.random(1,#files)) or "dummy")
        else
            context("dummy")
        end
    end

\stopluacode

\def\CollectGraphics[#1]{\ctxlua{document.collectgraphics("#1")}}
\def\ChooseGraphic      {\ctxlua{document.choosegraphic()}}

\CollectGraphics[*.png]
\CollectGraphics[*.jpg]

\starttext

    \dorecurse {10} {
        \expanded{\externalfigure[\ChooseGraphic][width=4cm]}
    }

\stoptext