The png files are only for still pictures, not for animated channels, so you will see them only in all mode except the Banner layout mode.
If your Wii is connected to Wifi, you can download artworks automatically for known game's ID by pressing button (1).
For homebrew which you created the animated channel yourself, you have to know the GameID and place the files in the corresponding folders.
If you don't, it's displayed on the List layout if you enable the GUI setting: display Both.