Hi Wolfgang,
Thank you for your suggestion, but I think I was not clear enough. In fact, I want to place the algorithm as the image.
Fabrice

\defineframedtext
  [algorithmframe]
  [width=fit,
   background=color,
   backgroundcolor=lightgray,
   framecolor=darkred,
   corner=round]

\definelines
  [algorithm]
  [space=on,
   before=\startalgorithmframe,
   after=\stopalgorithmframe]

\starttext

\startmakeup[standard][align=middle]

\startalgorithm
Initialisation :
  \math{u} prend la valeur 2\,000
  \math{S} prend la valeur 2\,000
Traitement :
  Pour \math{i} allant de 2 à \math{n}
     \math{u} prend la valeur \math{u\times 1,008}
     \math{S} prend la valeur \math{S +u}
  Fin Pour
Sortie :
  Afficher \math{S}

\stopalgorithm

\stopmakeup

\startlinecorrection[blank]
\startmidaligned

\externalfigure[cow.pdf]

\stopmidaligned
\stoplinecorrection

\stoptext