with this command:
~/edx-platform: paver update_assets cms --settings=aws
First, there was no file locates at themes/ourtheme/images/bg.png . Then what?
I took a tour into the template directory and check the sass file at /edx/app/edxapp/themes/unschool/static/sass/_ourthem.scss
There were one variable which was defined with wrong url or didn't been used anymore in the template:
So, I just comment out that var and run the compile assets command again. Everything's ok now.