Change Background on Digital Image

Is there any free software to download that would allow you to change the back ground on a pic out of digital camera. Example a bird flying on a cloudy day and you want to make the background a blue sky.

I doubt it, i'd use photoshop. Look at the similar threads at the bottom left of this page.

If you blew out the clouds to get the bird properly exposed... you can use gimp to change the white to blue...

Thankyou both for the good feedback, much appreciated.