NMC supports automatic logo painting.
Just place a image called "modulename_logo" .JPG, GIF, BMP, PNG in the skin folder.
The image is not resizing but supports transparency (just set the background transparent in GIF or PNG).
Module names are: main, audio, video, applications_externes, photos
Some module's automatic logo painting might not work (ive tested only the main module ) so report them to me and i'll fix this for the Alpha.2
NOTE: If you have some pink pixels while showing in NMC, it's because your source image is too "antialiased" (edges) and NMC is using Pink color (#FF00FF) to remove the background of the transparent image. So if you want to remove theses pink pixels, just use you favourite Image editor to remove some transparent pixels from around the edges. (hard for me to explain in english also

)
See the source Audi logo Image to see how i've made.
All in images:
If you set antialising on your logo when you want to have a trasnparent background, this is how NMC will show:
So, open your favourite image editor, change (temporary) the backcolor to red or another flashy color, and find the bad pixels causing NMC to show pink pixels, like in this image:
Other pretty invisible pixels are to delete, you can select the background of the image with the selection tool and show where are they, like in this screens:
And finally reset the background to transparent, save, and... WOW, this is working
In Future Dark skin:
In current (new) official skin:
And even in the old official skin, without making any changes !
So, let's make your custom car logos
Here's a link to find very good .AI (illustrator) vectorial car manufacturer logo's, you can import them in Photoshop or Fireworks, you just have then to delete pixels as i said.
Logotypes
Here's the ZIP containing the Audi Logo, you can use it to see how i've made:
Audi Logo
Here's the official thread to download logos:
Car manufacturers Logo thread