Is there a way of replacing the default 7-Zip icons?

Use 7Zip Theme Manager . You can change the file type look and feel as well the toolbar.

You have two options:

1. Changing the icon associated with zip files in the registry
2. Replacing the icon referenced in the registry

For the first option, you can edit the registry. The icon should be associated in HKEY_CLASSES_ROOT\zipfile\DefaultIcon. The entry is a string with the full path to the file (most probably the 7-Zip executable), and optionally a 1-based index of the icon number to use, seperated from the path by a coma. I know a tool that is helpful if you don't wand to fiddle with the registry manually: WAssociate

For the second option, I recommend ResourceHacker.

The easier way in my opinion is option 1, I recommend to go that way. You need elevated permissions for changes in HKCR, though.