Cannot change Default Thumbnail when editing Observation

[JoeCohen]

To replicate:

• Edit any Observation having multiple Images
• Click the radio button next to any Image for which the radio button is not already selected Note: There is no "Set as Default Thumbnail" button like tthe one that appears when creating an Observation. That's probably at least part of the problem.
• Save Edits Result: The thumbnail is unchanged

[JoeCohen]

The "Set as Default Thumbnail" and "Remove" buttons are in the form for adding Images (app/views/observations/form/images_upload/_template.erb). This works fine when creating an Observation. But when editing an Observation, these buttons aren't displayed for Images that are already in the Observation. (There's a normal radio button next to those Images, but it is ignored.)