Thanks for the screencast of the issue – very helpful!
I noticed in the ACF field settings for the before_photo field, you have the return value set to Image Array. With this setting, more than 1 value is returned meaning WordPress doesn’t know which value to place in the src.
You can change the return type to Image URL and this should give the expected result.
What an amazing year! 🎉 ACF grew by leaps and bounds in 2018 and won't be slowing down for 2019. We have lots of new features planned, but now it is time to rest, recover, and spend valuable time with friends and family. 👨👩👧👦🎄🎁