Exhibits: if you edit an banner with image attach you get a warning and can't save it

Bug Description:
If you create an exhibit with images. Go back and edit it and then go to save it you can't because the images field it empty.

SCA Version:
Django 1.3
Python 2.6.5 (r265:79063, Apr 16 2010, 14:15:55) [GCC 4.4.3]
preflight 0.1.1
SCA 12.09 r716
openid 2.2.1
pkgme-service https://pkgme-service.canonical.com/pkgme/api/1.0 {u'version': u'r113'}

OS:
64bit Precise

Browser:
FF 15.0

Problem URL:
https://sc.staging.ubuntu.com/exhibits/13/edit/

Steps To Reproduce:
1. Create an exhibit
2. Save it
3. Edit the exhibit display code
4. Save the exhibit

Expected Result:
I expect it to know that there are images tied into the exhibit and not tell me that the field is empty