Select the appropriate button: 'Move Selected Pages...' or 'Delete Selected Pages...'
Cleaning up your subsite image gallery
Go to 'Tools & Information' tab
Select 'Image Gallery'
Select designated subsite _______ and press 'show images'
Only images that are "public" or entered by the current user will appear
Delete each image by clicking the 'X'
NOTE: if you delete something that is being used, it will be gone from the page -- If it is being used it will give you a warning "This image is currently in use on one or more pages and/or elements."
In order to see which pages click the pencil icon below the image and choose: 'Referring Pages'