On 2013–02–05 Jaroslav Hajtmar wrote:

> I would like use modes alternative to change of image dimensions in
> different documents (presentation or print version).
> Compilation in ConTeXt TeXLive 2012 version will compile without
> error, but without visual effect. Compilation with the standalone
> version fails. What is problem or eventually. how to solve my
> problem another way.

How about this:

%% \enablemode
%%  [presentation]

\defineexternalfigure
  [narrow]
  [width=2cm]

\defineexternalfigure
  [wide]
  [width=7cm]

\doifmodeelse{presentation}
  {\useexternalfigure[COW][cow][narrow][height=3cm]}
  {\useexternalfigure[COW][cow][wide]  [height=3cm]}

\starttext
  \input tufte
    \startplacefigure [title=Caption, reference=cowref]
      \externalfigure [COW]
    \stopplacefigure
  \input tufte
\stoptext

Marco