static: Image optimisation
PNGs, GIFs and some other image formats can be optimised to make the images smaller without losing any quality. Smaller image files can be downloaded quicker by web browsers, and this can significantly improve user experience.
Images should be compressed before committing them to git. This can be done via:
bundle exec image_optim --no-pngcrush --no-svgo -r app/
This may require some extra binaries to be installed on your system - please read the output of the above command or see https://github.com/toy/image_optim for more details.