Identify images in your asset directory that may no longer be used.
install the gem
gem install redundant
run redundant from the root directory of your rails app to see which files may no longer be used.
Redundant works cross referencing the image files (jpg, png, gif &webp) in your assets/images directory and cross referencing the image filenames that appear in files in assets/stylesheets, assets/javascripts, helpers & views. If it doesn't find a reference it lets you know.
The results provided by redundant are a suggestion, i would advice you check wether you really want to get rid of them.