I've just installed the very handy image module - already like it very much!

I need to add a image size such that the image will be (max) X pixels wide. First I thought I could simply leave the heigth field empty but the only way I get it to work is to give a large-enough Height value (I use the largest possible 99999).

Is this the way it is suppose to work or can I achive this in a better way?