One way you can easily customize and change the look and feel of MediaPortal is by adding your own logos for:
You may also find many logo packs available for download in our Downloads Repository > Logos category
Be sure to check the wiki pages under Localization > By Country. Many users have added good sources for TV and Radio Channel logos there.
See Channel Logos for further information.
Skins generally provide Media Logos as part of the skin media files in the Skin folder \Media\Logos.
You can always replace these with your own logos, although make sure to back up your own logos so you don't lose them if you reintatll or update the skin.
Of course, if you change the skin media logos, you will need to recache your skin folder