Hello everyone and in particular to Wolfgang who has already provided me with the solution below. I will have to write a Mathematics book for my students and the remarks and graphics should be in the right margin. I got the code written by Wolfgang and tried to include the graphics in it. The result is not as expected because the graphic overflows on the text.
The course will be written in A4 format with an 11 pts font.
Thanks for your help.
Fabrice

\useMPlibrary[dum]

\setuppapersize[A4]

\setuplayout
  [width=12cm,
   rightmargin=5cm]

\definemargindata[marginfigure][inright]
\setupmargindata [marginfigure][command=\vbox,align=middle]

\defineuserdata [margintext] [alternative=margintext]

\defineuserdataalternative [margintext] [renderingsetup=userdata:margintext]

\startsetups [userdata:margintext]
  \margindata
    [inright]
    [%style=\smallbodyfont,
     style={\switchtobodyfont[9pt]},
     width=\rightmarginwidth,
     align={flushleft,broad}]
    {{\bf Remarque — }\getinlineuserdata}
\stopsetups

\showframe
\starttext
\startuserdata[margintext]
 \input ward
\stopuserdata
\marginfigure{\externalfigure[dummy][width=\rightmarginwidth]}
\startchapter[title=First chapter]
\dorecurse{2}{\input knuth}
\stopchapter
\stoptext