\setuppapersize[A8,landscape][A4,landscape]

\setuplayout[nx=3,ny=4,dx=3.5cm]

\setuplayout
  [height=middle,
   width=middle,
   topspace=3mm,
   backspace=3mm,
   header=0pt,
   footer=0pt]

\starttext

\startstandardmakeup
    \externalfigure[cow][width=\textwidth,height=\textheight]
\stopstandardmakeup

\startstandardmakeup
    \externalfigure[cow][width=\textwidth,height=\textheight]
\stopstandardmakeup

\stoptext
