[NTG-context] Problem inserting image within MP

[context at vivaldi.net]

Hello,

this is a probably basic question, but the following code doesn't 
compile:

----
\setupbackgrounds[page][background={Bkg}]

\defineoverlay[Bkg][\useMPgraphic{Bkg}]

\startuseMPgraphic{Bkg}
   externalfigure "hacker.png";
\stopuseMPgraphic

\starttext
   Ahoj
\stoptext
----

with:

"
>> picture
! Isolated expression.
<to be read again>
;
<*>  externalfigure "hacker.png";
;
I couldn't find an `=' or `:=' after the
expression that is shown above this error message,
so I guess I'll just ignore it and carry on.
"

I just want to insert a picture from within MP code. What am I doing 
wrong?

Best regards,

Lukas
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