For the design

Pick and prepare a picture A design. For the design, don't try to be subtle. You will get a slight height difference, but nowhere near what you may have hoped for.
For the design, don't try to be subtle. Grey shades don't carry particularly well: the most clearly articulated part of etched chocolate is the difference between etched and not etched: different shades will be hard to pick out, if at all possible, in the finished product.

For our running example today, I picked a picture of the Golden Gate Bridge I took a while back. The picture is already in black and white, so we can skip the greyscale conversion: however, I would recommend you do convert your image to greyscale this gives you more control over what parts of the image are dark or light. However, to emphasize the http://www.monclerstore.co.uk/ shape, I used Photoshop's filter to get a more quantized image with very few grey scale steps.