I don't understand why mediawiki.org would be easier to upload than
commons.
The design audit categories need to apply to existing mediawiki.org design pages as well as screenshots. mediawiki.org can reference screenshots from commons, but AFAICT it can't apply a local categorization such as [[Category:Inconsistent buttons]] to them. Also, Commons images have different norms, e.g. date is not part of the file name and people replace with improved versions.