I do not think this is possible...
To hide means to delete it or to rename it so that the image name starts with a . (dot), but you only have read access to images your app does not own...
You could take an image with your app or download it from the internet and display it only in your app without showing it in the gallery
EDIT: in case you do not plan to publish the app on Google Play, you can get manage external storage permission and delete/rename a file...