The short answer as to why your version isn't working is it's using Through Camera Projection but with no camera specified (switch to Plan Y), and the size is only 1x1 meter (the default for the Image Map Shader in the current public release; the next version will have a more intuitive default), and because Displacement is probably not enabled. Change the projection to Plan Y and the size to something like 1000x1000 (this is in meters) and you should see your image. As I mentioned, the next version will have better defaults.
You can use Pablo's method, it's a more direct route to getting height from it, but you may also or instead want to use your image as a mask instead of direct height, for which an Image Map Shader is useful. IF you use Pablo's method with a Heightfield Load node, then you can resize the heightfield easily if desired using a Heightfield Operator (added using the button below the Node List on the left in the Terrain layout, when your Heightfield node is selected).
Using an Image Map Shader would be more versatile as you could indeed enable Displacement, as Dune suggests (on the Displacement tab), and then get height out of it (the Displacement Multiplier is basically going to be your height in meters), and you can easily change size directly using the Size settings in the node itself without adding any additional resize node (again the size is measured in meters).
You could also use the same shader as a mask for other displacement shaders such as an Alpine Shader for craggy mountains, or a Power Fractal. The reason I say you may not want to use the image file directly as displacement is because your shape is going to be extremely simple and not realistic, it will turn out looking smooth and lumpy if used directly. If you instead use it to mask a noise function that is creating displacement, you'll get more natural variation. Unfortunately it's quite difficult to get the noise function to directly follow your input image, it is simply being masked by it, so it will have the same basic shape, but high (white) areas will not necessarily be high (i.e. if the "high" area in part of your input mask image happens to correspond to a "low" area in the fractal terrain you're masking). But it's certainly worth trying.
- Oshyan