How are explosions done? in 2d

I am starting a new game, and it is going to need big explosions for a 2d side scroller game. Just wondering - how are explosions normally done? I would imagine you could get a series of bitmaps (would have to have alpha on the edges) and just run them as an animation. But how do you "draw up" the frames for animation? I have photoshop and can fancy effects with it - but how do you do each animation frame?
The sort of explosion I want is the really nice looking ones that fade away. Hope that makes sense. Any tips appreciated.
The sort of explosion I want is the really nice looking ones that fade away. Hope that makes sense. Any tips appreciated.