Whenever you post an image on a question or answer, and it's not already hosted somewhere else, i.e. you're uploading from your computer, that image is hosted o

Whenever you post an image on a question or answer, and it's not already hosted somewhere else, i.e. you're uploading from your computer, that image is hosted on imgur. Stack Exchange has a deal with them like that. You can see this on the image upload dialog.