\definetextbackground [mybackgroundOverlay] [mp=mpos:region:whatever]

\startuseMPgraphic{mpos:region:whatever}
    for i=1 upto nofmultipars :
        draw textext("\externalfigure[cow.pdf]") rotated 10 shifted center multipars[i] ;
    endfor ;
\stopuseMPgraphic

\starttext

  \startmybackgroundOverlay
        \dorecurse{10}{\input knuth \par}
  \stopmybackgroundOverlay

\stoptext
