If the method returns a URL the template will look like this:
<img src="${view/get_portrait_url}">
In one project I have the following method so you probably miss the call to absolute_url method aswell:
def get_user_portrait(self):
portal_membership = api.portal.get_tool('portal_membership')
portrait = portal_membership.getPersonalPortrait(self.user_id).absolute_url()
return portrait