In resemblance of [AliasSettings]\AliasList documentation page, there should be a Confluence 5x page explaining how to create Image Aliases in 5x.

Namely, the settings that should be added to ezpublish.yml

image_variations:
		new_alias:
			reference: null
			filters:
			  - { name: geometry/scaledownonly, params: [100, 400] }
			  - { name: colorspace/gray }

It would be good also to make it clear if the new image aliases should be configured only in ezpublish.yml, or if they should exist also in legacy image.ini.append.php