Hi. K2 images are loaded by a relative path in order to work as they should both under http and https. The image that is not loaded is inside the content, it's not a K2 image. So you can go and edit the image to be whatever path you want. If you edit it but it still reverts to https then check the editor you are using for any relevant settings.
I'm using JCE 2.2.6 and I went through the Global Configuration and Editor Profiles but didn't see a setting that might be causing this issue. Would you be able to point me to a specific setting that I might investigate?