1.Per my test, external users cannot use Microsoft Forms service.
2.To let external users upload photos into a folder, you should enable external sharing both in the tenant level and site level.
(1)Enable external sharing in the tenant level. Go to SharePoint admin center -> Polices -> Sharing -> Set Anyone in the Content can be shared with section.
(2)Enable external sharing in the tenant site level. Go to SharePoint admin center -> Sites -> Active sites -> Select the site -> Sharing -> Set Anyone in the External shaing.
(3)Site permissions -> Share site only -> Share the site with external users.
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