This picture of a glowing cat has not been altered in any way. The cat, on the other hand, has been altered quite a bit. A team of researchers has genetically engineered it to express green fluorescent protein (aka "GFP," originally found in jellyfish), which makes the cat glow green under ultraviolet light.
This kitten may have the key to protect humans against HIV, the lentivirus that causes AIDS. He was genetically modified at the Mayo Clinic in Rochester, Minnesota. And yes, he glows in the dark.