Ever wondered what the hell WordPress was doing when you tried removing an annoying wordpress image, but it still appeared but with an i2.wp.com or i0.wp.com attached before the image? Simple. Your image is being used as a featured image somewhere.
I had this trouble recently while trying to update an image for a partner logo, only to be thought crazy when I deleted the image from my Media Library, insert a new URL, and still found that the image wasn’t being updated on the live page. The solution was right there, below the fold in the editor.
So remember, if you’re having trouble deleting an image, make sure it’s not set as a featured image somewhere else!