[NTG-context] externalfigure and width/height scaling

Hello, I am new to ConTeXt, so please be patient with my newbie questions. Looking at the \externalfigure capabilities, I haven't found any option allowing to scale only one dimension (or scale both dimensions in an anamorphous way). Am I right, and if so what is the best method to perform this? I thought about using the \getfiguredimensions macro to get the natural dimensions, and then use something like width=0.2\imgwidth,height=0.8\imgheight, but it is a pity that the externalfigure interface does not allow this directly (something like wscale and hscale options). Regards, BG