A picture's foreground can be cut out and composited onto a different background, making for interesting and otherwise impossible composites.
If a background is already white, simply make it transparent. See the article on isolating on white for more information.
To remove a background using the Eraser tool, first make the image as large as possible (you'll want either the horizontal or vertical resolution to be at least 2,000 pixels). If you're working with a smaller picture, blow it up by 200%, 300%, or even 400 percent. Using the Eraser tool on images with insufficient resolution will create edges that are too sharp. Also, any stray eraser movements will be all too visible.
After removing the background, you'll be reducing the size of the image again, which will smooth out any imperfections along the subject outline, as those pixels will be averaged.
If your image is already the right resolution, choose the Eraser tool and zoom in to 400%. Outline the subject you want to cut out using the eraser, being careful not to stray onto it. Pause erasing every few seconds in order to lock in your progress. That way, you'll be able to restore to where you had been using the Undo command in case you accidentally erase the wrong pixels on a given eraser run.
After performing an up-close outline of the object, zoom out to 200% and trace it again.
Now that the outline is complete and the background no longer touches the subject, zoom out to 100% and use the eraser to remove the leftover background.
Finally, use the Fill tool to replace the color white with green. This will make the never-erased parts of the image highly visible. Use the Pencil tool to remove any remnants of the original background that you may have missed.
Choose the Eraser tool again and press ALT. Fill in the green space with transparency. The picture will now be ready for photomontage with other graphics, like those on your homepage.