Updating existing event removes the image

[Atriusftw]

To reproduce:

1. Edit an existing event that has a banner image.
2. Optionally make any changes, do not touch the image field.
3. Save
4. Open the event again, now without an image.

It seems like the form is saved in the literal state it is at when editing. Since we are not uploading a new image, it'll remove the existing one because the value is blank in that field.