@Tony Lawn, Thanks for posting in Q&A.
For your issue, could you please confirm us some information? Did you create another wallpaper policy and assign to the same user group or device group? For the conflict error, it happens when different policies update the same setting to different values.
https://learn.microsoft.com/en-us/mem/intune/configuration/device-profile-troubleshoot#conflicts
Please unassign the first policy from the device and only assign the other policy to this group. Sync policy on device side and see if the issue can be fixed.
Also, here is a link you can refer to update the wallpaper with the same policy and will not cause conflict.
https://cloudinfra.net/how-to-set-desktop-and-lock-screen-wallpaper-using-intune/
Please check above information, if there is any update, feel free to let me know.
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