Typically, you can select the pictures from LPM and press the Delete button. Those will be deleted only from the application, but if you want to be sure that nothing else gets deleted, simply rename the folder with the pictures, then switch to LPM and delete all of them. Even if it tries to delete them from disk, the app won't find them because you've renamed the folder. It's simple and effective and this way you can be 100 % sure that the pictures on your hard disk will be untouched.