Ever wanted to import your images as Source materials? Here's how!
Official Valve Developer Community tutorial
Reflective materials tutorial
Note: When I say its width and height must be a power of two, I mean 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, or 2048. The width and height do not need to be the same.
Today I spent about five hours nailing down a section of Operation Metro from Battlefield 3 in the Source Engine. Here's what I ended up with.