This sounds easy - but is not: due to Plones broken-never-refactored portrait handling. Thus the only way to achieve it is to provide the image url in a separate field and customize the templates showing the portrait or ugly-patching of portal-memberdata and co.
see also plone/Products.CMFPlone#1263 (old https://dev.plone.org/ticket/12870 - never found resources to work on it)