wiki/layouts/_default/_markup/render-image.html

{{$src := .Destination | safeURL }}
{{$external := strings.HasPrefix $src "http" }}
{{- if $external -}}
<img src="{{ $src }}" alt="{{ .Text }}" {{ with .Title }} title="{{ . }}" {{ end }} />
{{- else -}}
{{$fixedUrl := (cond (hasPrefix $src "/") $src (print "/" $src)) | urlize}}
<img src="{{.Page.Site.BaseURL}}{{ $fixedUrl }}" alt="{{ .Text }}" {{ with .Title }} title="{{ . }}" {{ end }} />
{{- end -}}