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:
The Flatten Image command merges all of the layers of the image into a single layer with no alpha channel. After the image is flattened, it has the same >appearance it had before. The difference is that all of the image contents are in a single layer without transparency. If there are any areas which are >transparent through all of the layers of the original image, the background color is visible.
This operation makes significant changes to the structure of the image. It is normally only necessary when you would like to save an image in a format which >does not support levels or transparency (an alpha channel).