In addition to the actual photos, Windows Phones (Windows 10 Mobile and Windows Phone 8/8.1) create a thumbnail of that photo that will be displayed when you're viewing your galleries as a whole or without looking at the individual photos. This process takes usually happens in an instant when you copy photos to your phone, but try copying a lot of photos (over 500, for example) and you'll notice that not all photos will be immediately listed in your gallery view.
However, when it comes to deleting those photos, the device takes more time in deleting those photos. Notice that after deleting a photo (or many photos, or taking out the SD Card if your photos are there) they will still be displayed in your gallery view, but try tapping on any of those photos and you'll be brought to a "low-quality" view of the individual photo (that's the thumbnail or cached version of the photo). Actually deleting the photo deletes the thumbnail eventually, but not sure if taking out the SD Card will actually delete the thumbnail also since that's usually just a temporary state.
P.S.
For those who are still wondering what the "Others" in the Storage Sense app is, these thumbnails are part of it.