[NTG-context] Working with graphics

I am using graphics in my PDF document. First I used something like this: \midaligned{\externalfigure[graphics/priceList][width=1\textwidth]} Initially this worked good enough. But now I have both little pictures and pictures that are large in the y-direction. At this moment I change the width parameter manually. But I would like something like the following: - A picture is not stretched more as a certain value. (Say two.) - A picture is never to wide. - A picture is never to high to fill more as 75% of a page. Is this something that is possible? Also: is it a good idea to use \useexternalfigure, or is this only necessary when a picture is used more as once. -- Cecil Westerhof