I have a SharePoint list with employee names to include their emp id, email, Name, and photo - pulling from 'Person/Group'. In my app, I have a gallery and an image control with ThisItem. SpeakerEmail.Picture. All photos display great in web editor and desktop. However, they are not visible on my mobile.
Figured out a painful workaround for this -- I'm sure there is a much easier way but I pulled the jpg from the collection, saved to a sharepoint list and returned as a photo collection. For others looking for a solution to doing something similar, I would recommend utilizing a flow to or possibly graph api (?) -- not sure if that is an option but worth checking out.
This is a limitation with the current platform, I couldn't find any update though that they've fixed it. It's because of an authentication procedure hence it doesn't display it. The workaround was to put the image in a public folder and that worked for me.