To do that you will need to know how to use gimp/photoshop or similar programs.
You will also know how to convert files to the .dds format using The Compressinator, DXTBMP or the Nvidia Photoshop Plugin.

You can find tutorials on these subjects at the maniaplanet wiki[wiki.maniaplanet.com] and the maniapark tutorials section[www.maniapark.com].

So here is what you have to do for high res icons.
You will have to open your skins/cars zip file and create a new icon.dds with the size 512x512.
Do this before you upload the skin/car or it won't work.

Happy modding ;)