Hi @David Heinz,
Thank you for posting in Microsoft Q&A forum.
To change the Windows PE wallpaper, the file's resolution should be 1024x768 for best results. For more details:
prov-t-winpe.htm
But for the background of the task sequence, we can set the background position according to the link below.
https://learn.microsoft.com/en-gb/archive/blogs/cameronk/snazzy-osd-status-with-bginfo