I assume you know you can change the tool bar around. All you have to do is create a Eudora plug-in in ResEdit that contains the icons you want with the exact name of the menu items, mailboxes, etc or what you have in your tool bar. To create a plug-in, when you make a new document in ResEdit, change the creator to CSOm.
Same thing goes for mailboxes as well.
Even if the icon exists in Eudora, yours will override the default ones.