Is this only for your own use Naji? If so, you could probably store them in the App's ASD (App Specific Folder), but you would have to use the App to get them there - where are they before the get to your phone? You may be able to access them from the phone's images folder, or failing that, access them from the documents folder via an Extension:
A better solution, especially if the App is to be distributed, is to have the App download 'on demand' from a Server, or from a Google Drive. Using on demand requires the App to have an Internet connection (WiFi) and only the images required at a moment in time would be downloaded for viewing.
Finally, if you have your own Website, you could set the images in web pages and view the site from the App.