@Anders Lund, Thanks for posting in Q&A. Based on my researching, to upload teams background image via Intune, we can choose win32 or Azure storage blob. If win32 is not what you want, you can consider use Azure storage blob. Here is a link with more details for your reference:
https://www.workplaceascode.com/2020/04/17/distribute-custom-backgrounds-for-teams-via-intune/
Note: Non-Microsoft link, just for your reference.
Hope the above information can help.
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