Raising Into 3d

Ok. I am new new new to opengl.
I don’t need codes or nything.
Just an idea on how to approach this problems

Problem:
I have a 2D image with varying shades of grey, the whiter a spot the more it should be raised into 3D.

Please help me on how to approach this problem.
Thanks.

http://www.videotutorialsrock.com/opengl_tutorial/terrain/text.php