adCenter is starting a pilot program to test image ads for their Content Network, including MSN sites. Image ads can be purchased on a CPC or CPM model. You need to have your own IAB standard creatives to participate.
Then, follow these instructions, per the adCenter blog:
1. If you manage your account on your own, then call the 1-800-518-5689 support line and ask to be enrolled into Performance Improvement program and the Image Ads Pilot
2. If you are in a Managed Service program, you should ask your Account rep about the Image Ads pilot