Hi @Damien Watkins ,
Per my research, I don't think we could restrict user only uploading particular icons related to work.
All the custom images we add on this page have image size (pixels), and potentially file size (KB), restrictions, however there is no specific type limitation.
Since only Global administrator account could custom the Company branding, we suggest you notify the Global administrator that don't upload logos which are not related to work.
Reference:
https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/customize-branding
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