Remove the image background of a subject, with Instant Mask
Instant Mask is a free, quick and easy to use program that will remove the background of a picture.
The project now has turned commercial, but you can try this freeware version and see if it meets your needs.
It gives you two brushes to let you define roughly the background area to be removed, and the foreground area you’d like to keep – like when, e.g. you’d like to keep a bird and remove the sky behind the bird.
Instant Mask is specialized to that task, and it works even better than some painting programs which include a magic wand to perform a similar operation.