1. Open MDT (DeploymentWorkbench)
2. Right Click on the Deployment share and select properties
3. Click the Windows PE x86 Settings Tab (Or 64 Bit tab depending on what you are changing)
4. Under Windows PE Customizations, you should see a Custom background bitmap file box. Browse to your image you want to use
5. Hit Apply and then OK
6. Repeat for the other Windows PE tab (if needed)
7. Completely rebuild your deployment share. I’ve never had luck doing just a quick rebuild. If I do a quick rebuild, my image never changes it stays the same as it was.

I also kinda remember the image needing to be a certain size I’m adding to my  WIMs are images that are exactly 1024*768*24 bit bmps and they seem to work just fine.