pngcrush not found

[dog1993]

When I Create the databases using"bundle exec rake db:create",it reports some errors like

“pngcrush not found,please provide proper binary or disable this worker（—no--pngcrush argument or‘：pngcrush →false’ through options ）”.
“advpng not found,please provide proper binary or disable this worker（—no--advpng argument or‘：advpng →false’ through options） ”.

It seems that these errors are related to image.Does the file "openstreetmap-website" on https://github.com/openstreetmap/openstreetmap-website.git lack some useful things?How can I fit it？

[tomhughes]

Duplicate of #1931.