You can change nearly any desktop icon on both Mac and PC. To change desktop icons on a Mac, open the "Get Info" menu and paste in the picture you want to use. On a PC you'll need to make a .ICO file, ...

I have a lot of icons in png format that I want to use with my X desktop. Unfortunately, they are waaay to big, about 128x128. I was wanting to know if there is a way to resize these icons, over 100, ...