In SfB on premises you have the ability to set “EnablePresencePhotoOptions” to true under New-CsClientPolicyEntry.
This then allows you enable users with the ability to specify a URL for a profile picture in the SfB client.
In SfB online, you’re unable to run New-CsClientPolicyEntry, and/or can’t enable EnablePresencePhotoOptions.
A a result you cannot specify a publicly accessible image URL for profile pictures and meetings with user external to your Office 365 will be presented with the default blue avatar picture.