It is possible to lockdown what backgrounds can be set in Microsft Teams via policy. The following PowerShell can be used with the Skype for Business Onine Windows PowerShell Module installed:

Import-Module SkypeOnlineConnector
$sfbSession = New-CsOnlineSession
Import-PSSession $sfbSession

# Lockdown backgrounds policy. Possible values are: NoFilters, BlurOnly, BlurAndDefaultBackgrounds, AllFilters.
Set-CsTeamsMeetingPolicy -Identity Global -VideoFiltersMode BlurAndDefaultBackgrounds

# Apply to all policies
Get-CsTeamsMeetingPolicy | foreach { Set-CsTeamsMeetingPolicy -Identity $_.Identity -VideoFiltersMode BlurAndDefaultBackgrounds }

This can prevent users from selecting inappropriate images as backgrounds in meetings.