Alpha channels (alpha planes)
Rapid Inspector will report a ALPHA_CHANNEL error when it finds an image with extra color (or channel) information. This error is inherent to an image that contains more color channels (or planes) than needed.
This problem should be addressed by opening the image in an image editor, such as Adobe Photoshop, and removing the extra color channels. To learn more about color and channels within Adobe Photoshop, consult the Adobe Photoshop manual or search for help on removing Alpha Channels.
More Information about Alpha Channels