You could calculate this manually,

i.e.:

function GetWidth(newHeight,orginalWidth,originalHeight)

{

if(currentHeight == 0)return newHeight;

var aspectRatio = currentWidth / currentHeight;

return newHeight * aspectRatio;

}

Make sure you use the ORIGINAL values for the image otherwise it will degrade over time.

EDIT: example jQuery version (not tested)

jQuery.fn.resizeHeightMaintainRatio = function(newHeight){

    var aspectRatio = $(this).data('aspectRatio');

    if (aspectRatio == undefined) {

        aspectRatio = $(this).width() / $(this).height();

        $(this).data('aspectRatio', aspectRatio);

    }

    $(this).height(newHeight); 

    $(this).width(parseInt(newHeight * aspectRatio));

}