Image resize by width only

[eluser]Random dude[/eluser]
I want my users to be able to choose from 3 sizes of images:
small (200px width), med(500px width), large(800px width).

I'm trying to use the image_lib to re-size images by setting width only,
for example, if a user selects 'small', and uploads an image that's 1000px/900px,
the image will be re-sized to 200px / (height adjusted automatically). In the 1000px/900px example, that would 200px / 180px.

Right now if I set width to 200px, the height of the image stays the same. For example, 200px/900px. I'm not able to set height because users will be uploading many images of completely different sizes.

What should I do?