Unlike \starthanging..\stophanging which behaves in the same way in both body text and footnotes, \placefigure does not. In footnotes, \olacefigure adds an opening blank line and runs the text through the image. It does, however, seem to place the image better. An example:
\setupexternalfigures[location={local,default}]
\starttext
text text,%
\footnote{
\starthanging[location=right]
{\externalfigure[cow][width=.1\textwidth]}
\dorecurse{2}{\input ward}
\stophanging}
and more text%
\footnote{\placefigure[right,none]{}
{\externalfigure[cow][width=.1\textwidth]}
\dorecurse{2}{\input ward}
}
\placefigure[right,none]{}
{\externalfigure[cow][width=.1\textwidth]}
\dorecurse{2}{\input ward}
\stoptext