It is a fake-picture! The hat was badly implemented through Photoshop... here is the big version (looks very bad). I loaded it up on my webspace, so please download it fast for that i can delete it again.
There's another thread showing a bigger version of that photo, but I don't remember which one. Anyway is not fake... because there are 2 pictures of Harrison dressed like that... you can find them at www.harrisonfordweb.com