When you go into your photos, opens photo and tap the share button (that’s the box with an arrow going out of it). Does your work app come up as an option?
In my experience, usually when an app wants access to your photos it asks for it...meaning you either open the app and there’s a popup dialogue box with ’allow this app to access....’ or when using the app you try to attach something and the app then asks ‘can this app access your.....’
It makes me wonder if your app is faulty. Does the app have a support page? Somewhere you can go to see if they have a support forum or site?