I have the exact same problem with the custom image field (ACF v4.3.8) on the posts edit page and the Infinite Scroll plugin (v2.6.2 by Beaver6813, dirkhaim, Paul Irish..) on WP 3.9.1.
Disabling the Infinite Scroll plugin resumes normal operation to the custom image field.
Screenshots to reference problem:
http://imgur.com/KAZbQtI
http://imgur.com/aV3Tl6U