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Open the App Store. Tap Search. Tap the search bar. Type "Photo Investigator" into the search field. Tap the "photo investigator" option. Tap GET. Tap INSTALL. Enter your Apple ID and password. Open the Photo Investigator app. Tap the photo icon. Tap OK. Tap All Photos. Select a photo. Review the "File Size" value.
Do so by tapping the blue "App Store" icon on one of your Home screens. This is on the bottom of your screen. It's at the top of your screen.  It should be the first result in the drop-down menu. This should be to the right of the "Photo Investigator: View, Edit, Remove Metadata" heading.  Your download should commence. It should be visible on one of your Home screens. This is in the bottom left corner of your screen. This will allow Photo Investigator access to your photos. You can also tap a specific album on this page.  This should be on the default Photo Investigator tab that's open below your photo. This value will likely be measured in megabytes (MB).