Hi,
AFAIK, the strel class has already been implemented but is not released yet. To obtain the most recent image package files, you'll have to clone the octave-forge svn repo (see
octave.sourceforge.net/developers.html ). Then you can contribute svn patches through octave's patch tracker (
http://savannah.gnu.org/patch/?group=octave) prepending the subject of your patch with the name of the package of concern (e.g. "image package: some new fearure").