Rails 4 images not loading on heroku

Rails 4 images not loading on heroku

I have spent the better part of the day trying to get images to load on my
heroku app. Everything I try works locally, but not after being deployed
to heroku.
I have png files saved in the images folder under my assets. I am
referencing these images with syntax in my css such as;
#signin{background: url(<%= asset_path 'sf.png' %>);
background-size: 100%;}
In heroku when I inspect the background the assets/sf.png link is there
but when you click it it shows a broken image, suggesting it did not load
properly.
I've tried toggling 'config.serve_static_assets = false' in the
production.rb file between true and false and neither works.
I also have group :production do gem 'pg' gem 'rails_12factor' end
Precompile is always successful.
Rails 4. Any ideas on what else to try?