This happens if the event got first liked, when it didn't have an image.

The default Events image will display in this case. This means that even after an image is added to your event in Events, Facebook is not updating it automatically.

To force Facebook to see the image that is on your event's page, enter that event's description page link (the one that contains eventsignup.asp) in the provided URL field on this Facebook debugger page then hit debug:

http://developers.facebook.com/tools/debug