All that's required to make a photo 'postable' is to allow it to be accessed via an Internet-exposed URL.

This may not be accomplished by 'cloud' storage that maps to proprietary interfaces for security, or requires 'credentials' or cookies to be able to access the content.

Here is the official Microsoft document on how to configure the consumer version (which I ASSume is what you have) and change any necessary settings:

(I don't use OneDrive actively, so this is provided without further comment on its contents)

The Forum software is so old that I doubt it requires https:// protocol, as modern browsers do, in the URL that references the image file.  Even valid http:// URLs with imperfectly updated site 'certificates' may cause a 'Danger, Will Robinson!' sort of security warning that treats access to the photo no differently from phishing attempts or cracking...