Using the AssetThumbnailURL tag it is perfectly possible to create a thumbnail of any image asset in Movable Type, with any required height or width. If you set only the height, the width will be rescaled proportionally, and vice versa. But what if you need to set both the height and the width (for example because you want to display a nice, rectangular image in your sidebar and the rectangle has to be completely filled) yet you don't want the resulting image to look too stretched or compressed?