When a user(who has the correct permissions) accesses the add in directly they are authenticated, but they are not able to see the inserted images. If they have previously accessed the site collection or teamsites within then access the add in they see the image.
I understand this is somehow related to the add in authenthntication vs the sharepoint authentication but wondering is there a way to solve this by still using the "img src" approach.
HI Damien I would reach out to the 3rd party app for their advice on this. Many 3rd party apps send you to their SharePoint Online environment which then "reads" into your environment which is likely the cause of the issue.. the user is technically no longer in your environment while using the app so the image source is not recognized.
I'm not sure what the add-in does, but if you need help or another solution, my company specializes in these types of things. Hope this helps!