DrakNYC asked in this post on the Pixelmator forum how to create this embossing effect. Here is my answer to that:
And here in words:
Step one is to create a background (in my case it's #cdcdcd) and give it some noise (Filter>Stylize>Noise…).
Load your picture of what you wish to emboss into the background as a new layer (it must be a black layer, in my case I use just a 'N') and change it transparency.
Now load a selection from the 'N' by cmd-clicking the layers icon in the layer palette.
Go to Edit>Stroke (cmd+shift+O) to add a centered white and black stroke of 1px.
Now we will have to erase some parts of both strokes. If we imagine a light from above, we will have to erase every white stroke which lies on each border to the bottom. Then erase all parts of the black stroke where the white stroke remains.
Change the opacity of each stroke until you get a good result.