The Add Picture control has much greater functionality than the Camera control and taps into all the inherent settings of the device’s camera capability such as the flash, timer and many other settings
Additionally, instead of just taking photos using the device’s camera, as an alternative, existing photos can be attached too. The photos can be sourced from the device’s photo gallery or from other cloud locations that the device can access such as OneDrive
The capability to attach photos is set by the UseMobileCamera setting of the AddMediaButton control within the Add Picture control. If this is set to false, which it is by default, then the Add Picture control can be used either to take a photo or add an existing photo. If the UseMobileCamera property is set to true, then the Add Picture control can only be used to take a photo and an existing image cannot be added
At the time of writing, my experience is that setting this property to ‘true’ only takes effect on IOS devices. Users of Android devices will still be able to add an existing image whatever the setting of UseMobileCamera. This can be problematic, for example if you wanted to mandate that users take a photo while on site, and not allow them to upload an existing gallery photo instead
If users will be running your app in a browser on a PC or laptop, then be aware that the Add Picture control is not an option. Yes, users will be able to add an image from file (if you allow them to), but they won’t be able to use the PC or laptop camera. This is also true if the app is loaded using the Power Apps desktop app. If you want the app to use the PC or laptop’s camera then the only option is the Camera control and not the Add Image control