Cutting out images from a background picture can be a difficult thing to do when the actual picture is very detailed. I actually use photo shop to cut around detailed pictures and to remove backgrounds. This allows me to use the image on any background I like. I can move the image around by itself and it has a transparent background.
Of course you could probably do this on a free image editing software such as gimp, however I have only used Photoshop for this detailed work. When I use Photoshop I use the Polygonal Lasso Tool for cutting the images from the backgrounds. I have tried the Magnetic Lasso Tool but it only works on projects with a clear color distinction.
For example it will not work when you are trying to cut out images that have similar color to the background already. Therefore I need to use the Polygonal Lasso Tool.
Once you have cut the image away from a background you are free to do what you like with it. It is great for people making banners or doing website graphics.
Another cool thing you can do is blend the pictures using photo fuse in Windows Live Photo Gallery. It is truly amazing what the average computer user can do with a simple photo now days.