Resize image to fit in box

[tobiasBora]

If I have two kinds of images with the same aspect ratio but some are in landscape mode and some are in portrait, I can't find a way to resize them so that they fit in a given NxN square. When using w=XXX, the height might be significantly larger or smaller (depending on the aspect ratio and portrait/landscape). Similarly, I might want a box to fit in a picture to be sure it is big enough. So it would be great to have something like:

atMost=500x500

and for the other option:

atLeast=500x500

Am I missing an obvious option?