Yes that is AI-Art. but that screenshot has a old bug in it.
That was fixed. (WotLK 2 point shields Cataclysm 4 Point shields)
So it looks better now.
AI-Art can be found here
If you only want the Achievement background you will need UI-Achievement-Alert-Background.blp (but that would look a bit different because off the shield (where you points are shown)
UI-Achievement-Shields.blp
UI-Achievement-Shields-NoPoints.blp
are the 2 for the shields, but they change it in the Achievement window as well.
All files are found in the package in the AchievementFrame folder