@AlexBlanes
First, just as an FYI...if it is a user photo you're trying to store, are you aware that each user already has a user photo? There would be no need to store that in SharePoint as it is already there and associated with their profile.
That said, if it is not that scenario, what column type is your picture stored in? If it is an Image column, then you will find that the image column type is not currently supported in PowerApps.
You can use Image/Hyperlink columns in your app, but they work a little differently.
Beyond that, you are in a quandary of trying to do something with storing base64 text of the image in a multiline text column.
I hope this is helpful for you.