Icons for your computer are ".ico" files, so any of the dock icons that are that file format can be used.

Any dock icons that are ".png" file format, will need to be used as an image to create an icon (".ico") file with. You can use Icon Developer to create these ".ico" files from a ".png" image (which can be downloaded here https://www.stardock.com/products/icondeveloper/downloads.asp)

As for changing the default icons on your computer, it will depend on which icons you want to change. There are a vast amount of icons included in many files in Windows (explorer.exe, shell32.dll) to name a couple of popular ones. In this case, you would need to either use a program like ResHacker to open and change the icons, then swap out the original file for the hacked one, or use a program like IconPackager - which will do the work for you, and is capable of changing many (not all) of the system icons, as well as any file icons that you wish to change.

I advocate using Icon Packager, due to the following:

1 - There is no hacking of system files needed - therefore, no chance of completely mucking up the operating system.

2 - IconPackager integrates into a file, folder, or drive's properties dialog, which allows you to change the icon image easily on an individual basis.

3 - The full (enhanced version) includes Icon Explorer, which allows you to manage and view icons.

4 - IconPackager themes are offered here at WC for download - from a limited amount for free, to unlimited amounts if you purchase the full version.

5 - IconPackager offers a package builder, which allows you to drag-n-drop an icon (".ico") image onto the icon you want to change, and then you can save your own theme as package to apply at once.

If interested, you can download the free version for a 30 day trial, or purchase the full version via the page here https://www.stardock.com/products/iconpackager/

I hope this answers your question.