I also don't have Word so I can't test directly, but the loss of antialiasing suggests that EMF may be in use. It's possible Word is placing "newer" EMF+ data on the clipboard, with old-style EMF included as a backup. (I say "newer" because EMF+ has been around since XP days... so not that new.)

While you could use some clipboard APIs to manually figure out what formats Word is sticking on the clipboard, as others have suggested, that won't help you solve the problem of the Rich Text Box picking whatever format it pleases.

My only remaining guess would be something messy like subclassing the clipboard, intercepting paste messages before they hit the Rich Text Box, and possibly sending your own EM_PASTESPECIAL or EM_CANPASTE messages until you find a combination that maintains image fidelity. That's not a project for the faint of heart, however.