My own height map is not working...

I'm new to height maps and that stuff, so excuse me a bit..
I'm using a relief mapping shader, and it works really great. But, when trying to make my own height map it just scales the whole texture when I change the height parameter. When using the Unity default particle texture as height map, the height parameter acually works as it should. So it's def something wrong with my height map..
I'm just using Gimp to make a black/white layer (same dimensions and stuff) and exports that layer.
How am I supposed to create my height map from my texture?

 

Are you using Unity 5 ?

Just black and white is not enough really for a good heightmap. You need grayscale. I often draw the darkest areas black and then use a gausian blur often it. Fast and ok results mostly for structures like walls.