This query will find most posts that contain at least one image that isn't hosted either on i.imgur.com or i.stack.imgur.com. Note that some images have legitimate reasons for being hosted off-site - maybe they're SVGs (not supported by imgur), or are larger than the imgur upload size limits (4 MB non-gif; 2 MB gif).
Also, the [Snippet] column may list an imgur URL; this is because it just lists the URL of the first image it finds. There may still be non-imgur images elsewhere in the post.
Q&A for enthusiasts and power users of the Android operating system
b.Id AS [Post Link],
ISNULL(b.ClosedDate, a.ClosedDate) AS ClosedDate
LEFT OUTER JOIN Posts a
ON b.ParentId = a.Id
b.Body LIKE '%<img%'
AND b.Body NOT LIKE '%<img src="http://i.stack.imgur%'
AND b.Body NOT LIKE '%<img src="http://i.imgur%'
--none of which are hosted on stack.imgur or regular imgur