I believe I also have somekind of similar graphic weirdos, but is in the game, not at the login page.

When I flying over the El-something forest outside Stormwind, I found all trees are bared with leaves! no leaves! All bald!

I guessed its due to me after tuning the monitor display frequency to higher (80 hz). Later I drop to 75. No more bald trees encountered.

Just guessing.