resources.Copy

{{ with resources.Get "images/a.jpg" }}
  {{ with resources.Copy "img/new-image-name.jpg" . }}
    <img src="{{ .RelPermalink }}" width="{{ .Width }}" height="{{ .Height }}" alt="">
  {{ end }}
{{ end }}

The relative URL of the new published resource will be:

/img/new-image-name.jpg

Use the resources.Copy function with global, page, and remote resources.