Timeline background image aspect ratio not preserved when using auto mode for top/bottom boundary

[saekow]

Timeline height in auto mode for top/bottom boundary appears to increase when a background image is set.

1- Here is "Timeline A" that I would like to set a background image:

Location of this test timeline:
http://www.chronozoom.com/Roland#/t40977d19-48bb-4a6d-b4ef-00fcb2f0cde8/tbde512ef-b96d-441b-a5da-555e6b4e0e24

2- Since I won't add any more sub timelines or exhibits inside this timeline, the aspect ratio of this timeline will be fixed. So to make sure I apply a suitable background image with the same aspect ratio, I take a screen shot cropped to the timeline borders and paste it into Photoshop:

3- I paste a photo as Layer 2 on top of the screenshot on the same canvas:

4- I save the background image and upload to imgur: http://i.imgur.com/ZJDhe9O.jpeg

5- I paste the imgur URL into the background image URL field:

6- After I update the timeline and refresh the page, the background image has been applied, but the aspect ratio is not preserved. The timeline height got increased in auto mode even though no additional subtimelines or exhibits were added:

Here is the input background image for comparison.
Since the background image was created with the same aspect ratio as the timeline, the expected behavior is the image should not be distorted.

Observed: The timeline height increased once a background image URL was applied.