As per the Wikipedia "alpha compositing is the process of combining an image with a background to create the appearance of partial transparency". To remove the transparency or the alpha channel:
- Fire up GIMP
- Open the image for which the alpha channel has to be removed
- Click the Image menu on the image window
- Click Flatten Image from the pull down menu
In short, remember to flatten the image when you want to remove the alpha channel.
Quote from the GIMP docs: