Yes, it is. I usually start writing a new answer, upload the image there, and cut/paste the link to the comment. Stack Exchange has its own 'channel' on imgur (that's why there is .stack. in the URLs), so you can count on that even if imgur somehow would stop hosting images in the future, Stack Exchange will migrate them automatically to a new hosting provider. Also, users in a corporate network where imgur and other image hosters are blocked sometimes have convinced their administrators to make an exception for i.stack.imgur.com since that is vital for Stack Exchange.