I ran into a problem where continue did not work properly with an imageinfo query:
The first query returns this : {"continue":{"iicontinue":"Barack_Obama.jpg|20081103215509","continue":"||"}
The second query returns this: {"continue":{"iicontinue":"Barack_Obama.jpg|20080913075003","continue":"||"}
After that, using the second continue returns the same data and continue value again (and again and again).