Well...literally, find a stock photo of flame. It's probably what they did.
Otherwise, if you'd like to generate it yourself, I'd have a look for tutorials on painting digital smoke or flame (although I just did a quick search and didn't find one that looked quite right). Alternatively, a measure halfway between cropping a photo and generating it yourself would be to use some custom brushes, for example,
maybe these.
That particular closeup of fire on the car also looks an awful lot like some of the energy-ribbon style desktop wallpapers I've seen... There's even a web-app called
Flame Painter that can very quickly generate those types of images.