This image is created using two photoshop brush sets that I made back in high school for a design project. They were originally uploaded for my friend to download and use, however unexpectedly they were quite a success online.
I initially uploaded them to deviantart where it became featured on a number of blogs, used in photoshop tutorials and was rehosted at numerous free photoshop brush websites. Across the two brush sets and different download sites they have been downloaded around four hundred thousand times.
They have been so prolific across the internet that if you type 'FlightOfGrey' (the online pseudo name I posted them under) then Google even auto completes that to FlightOfGrey stroke brush set.
The brush strokes were strokes of shellac. Shellac is a semi transparent thick resin, which meant that a lot of the details about how the strokes and material interacted is still visible. The brushes themselves were also at the maximum resolution for brushes in Photoshop CS2 which I assume also helped with their popularity.