\showgrid
\setuplayout
  [header=0pt, footer=0pt,
   topspace=0pt, bottomspace=0pt,
   height=middle, width=middle,
   backspace=0pt]

\starttext
  \startluacode
    require "lfs"
    function GetFileExtension(url)
      return url:match("^.+(%..+)$")
    end
    cwd=lfs.currentdir()
    for file in lfs.dir(cwd) do
      suffix = GetFileExtension(file)
      if suffix == ".png" then
        image = figures.getinfo(file).used
        width = image.width or 0
        height = image.height or 0
        aspect = height/width
        if (aspect > 0.9 and aspect < 1.1 )  then
          context.externalfigure({file},{width="\\textwidth"})  -- Almost square
        elseif height < width then
          context.externalfigure({file},{width="\\textheight", orientation=90}) -- landscape
        else
          context.externalfigure({file},{height="\\textheight"}) -- portrait
        end
        context.page()
      end
    end
  \stopluacode
\stoptext
