The quickest way to remove a page from Google is to 410 the page. Second quickest being a 404.
When you delete a page on Wordpress (or any website), the page will automatically turn into a 404. Google will crawl the page a couple of times, and within a few months, the page will be dropped. Whereas with the 410 response, Google will crawl it, and instantly delete the page from their index. So it depends on how quickly you want them gone.
As you're using Wordpress, I'm sure they'll be many plugins which can do this for you. If you're techy, and comfortable with htaccess, you could also simply add:
RewriteRule ^gallery/stockphoto-123/ - [L,R=410]