To change the icons of folders like My Pictures and My Music,
1. Go to "Folder Options" in the Control Panel.
2. Go to the "View" tab.
3. Select "Show hidden files and folders", uncheck "Hide protected operating system files", and click OK.
4. Go to the folder which you want to change the icon of and open the "desktop.ini" file.
5. Change the value of "IconFile" to the file path of the icon you want to use.
6. Change the value of "IconIndex" to "0" if the icon file has one icon. If the icon file has multiple icons in it (like SHELL32.dll), you will have to change the number until you have the right icon.
7. Save the desktop.ini file when you are done.
If you are looking for icons, you could try
Icon Archive.