I've never done this, but I would start by changing the images in the ZD server's directory (not the user part, beware) which is under
$ZD_root/jetty/webapps/zimbra/img/zimbra/ there you would see all the icons zimbra uses for this (there are other directories as well) in PNG format. I'm using ZD 7.0.1 and I have 365 files there.
Now, these images get merged into a file called
$ZD_root/jetty/webapps/zimbra/img/zimbra.png which, as I understand, is from where ZD actually gets them. I don't know how to trigger the merging process, but perhaps someone else can answer that....or you could just modify the merged image
It may take a few cache clears on the client (Prism) side for the changes to show up.