How can I turn a grayscale image into a .stl 3d file?

I have an grayscale image, I'd like to export it as an .stl file for 3d printing using the function surf2stl. I have been trying to turn the image file, which is a two dimensional matrix, into a surface using the color values as a height. Using the function surfht, I can get the interpolate contour map I want to print, but it is only displayed and I can't use it in surf2stl, also the surf function returns a 1x1 value, which can't be right. Is anyone able to shed some light on what steps I should take to turn this image to a surface and then an stl, or what I am doing wrong?

Thanks!

surf2stl code: http://www.mathworks.com/matlabcentral/fileexchange/4512

The image I'm using: http://imgur.com/cDszY