I recently tried a bit of the double-exposure effect. This works quite effectively when using two silhouette type images. I wanted to start out with this photo though:

That took a bit of photoshopping to invert. Then I took a smartphone shot I took while running of a tree near my home and put that on top. The basic idea is to have a regular background image (the head in this case) and overlay the other silhouette image above that and change the layer blending mode to ‘Screen’.

In practice, I used quite a bit of masking to get the effect constrained to the areas I wanted to. This is what I ended up with:

Thanks to Chanelle for posing in the original image!