Hmmm... What happens if you select one of your contacts that is both a Facebook contact and a Google Contact, open it up and then unlink the 2? Does one of them have a picture now? That might explain something, and point to a bug.
If you don't see anything interesting there, I would:
1) Delete the Facebook account.
2) Go into settings > accounts and sync > select your Google account > uncheck Sync Contacts.
3) Download Contact Remover from the Market.
If the following step makes you nervous, you can export your contacts to your SD card via the Contacts app. I have done the following before myself, without loss of contacts.
4) Open Contact Remover and Select All (Your Google Contacts will still be listed).
5) Go back into settings and reselect Sync Contacts in your Google account.
6) Wait until all contacts have finished syncing.
Do they have pics?
7) If yes, open Contacts.
8) Click the menu capacitive button.
9) Click more then click Get friends.
10) Choose Add Account and finish from there.
Have pictures for everyone else now?
The integrated Facebook app is not known for it's ability to work well when it comes to syncing. For reliable picture syncing, I recommend an app called syncmypix. It's free and works well.