You use a resource editor like Restorator or Resource Hacker 3.5 Beta.
You go to Shell32.dll in your System32 folder. If you use Restorator you Extract the bitmaps to your desktop. Open the bitmaps in your image editor. Save your changes. Now copy and paste your new bitmaps on to the old ones via Restorator. Save as Shell32.dll to your desktop. Now you have a new Shell32 with your bitmaps.
I would also do bitmaps 632 & 633 which are the left panel in the Control Panel screens.
The bitmaps must be 32 bit bitmaps. I usually convert the bmps to pngs using an online convertor. Then I save the pngs in MSPaint which will automatically convert them to 32 bit pngs. Then I go back to the online convertor and convert the pngs back to bmps. Now I have 32 bit bitmaps. There's probably easier ways, but this way works.
This is the online convertor I use:
http://www.coolutils.com/Online/Image-Converter/