Open the MDT Deployment Share Properties
In the second tab “Windows PE x86 Settings & Windows PE x64 Settings do you this setting.
For example in the Windows PE x86 Settings

In the Windows PE Customizations do you find the Custom Background Bitmap file:
The default image location is: %installdir%\Samples\Background.bmp
Actually the real location is: C:\Program Files\Microsoft Deployment Toolkit\Samples
In the sample directory, rename the Background.bmp file to Background_old.bmp
Copy the new backround file to the samples directory with the original file name (Background.bmp)
Update the Deployment Share and mark also the regenerate boot image option
Update the WDS bootimages and/or the LTI Bootable Media
The background wil now appear when you run an new deployment.