OK, so here are a few basic concepts you need to get your head around first off. For starters, most channels are stored as "WAD" files. These are essentially archives (a bit like a ZIP or a RAR) that hold all the channel-y goodness. Inside a WAD, you will find, along with a few unimportant files for this tutorial, files called 00000000.app, 00000001.app and 00000002.app. The "banner" of the channel is in 00000000.app. This file is another type of archive, this time a "U8 archive". Inside, the structure is like this:
sound.bin is the sound that plays when the channel is selected. icon.bin is the version of the banner displayed on the System Menu. banner.bin is the version that is displayed once it is selected.
banner.bin and icon.bin have a very similar format, like this:
The .brlan files handle the animation of the banner, the .brlyt files list all the "textures" (images) used and the .TPL files are themselves the textures (images) used within the banner.
It follows that by simply replacing the .TPL files, you retain the animations of the banner, but change the images. And that is the first type of banner creation most people try, and details of that can be found here: Simple Banner Creation
Here you use what are known as "Base WADs". These are WADs made by other members of the community using completely legal tools, which can be modified legally to create custom channels. The creators of these banners have made the images and animations in use within the channels.