When referencing Peter Corke's code for iblobs it appears that it generates a vector of Region Features, where plot_ellipse is an object of that class. Therefore, the ellipse needs to be defined before plotting it.
Set major and semi major axes as options in "iblobs" as well as the orientation of the ellipse. Please refer again to Peter Corke's iblobs page in which he defines the aforementioned options. I hope this points you in the right direction.
