Code snippet used in https://adityatelange.in/blog/hugo-watermarking-images/

render-image.html

<!-- contents of render-image.html -->
{{- $link := split .Destination "#" }}
{{- $image := (.Page.Resources.ByType "image").GetMatch (printf "*%s*" (index $link 0)) }}
{{- $logo := (resources.Get "images/logo.png") }}

{{- if and $image $logo }}
{{- $size := math.Round (mul $image.Height 0.25) }}
{{- $size := cond (ge $size 80) ($size) (80.0) }}

{{- $logo := $logo.Resize (printf "%.0fx jpg" $size) }}
{{- $image := $image.Filter (images.Overlay $logo (sub $image.Width $logo.Width) (sub $image.Height $logo.Height) ) }}
{{- $finalUrl := cond (isset $link 1) (printf "%s#%s" ($image.Permalink) (index $link 1)) ($image.Permalink) -}}

<img src="{{ $finalUrl | safeURL }}" alt="{{ .Text }}" {{ with .Title}} title="{{ . }}" {{ end }} />
{{- else }}
<img src="{{ .Destination | safeURL }}" alt="{{ .Text }}" {{ with .Title}} title="{{ . }}" {{ end }} />
{{- end }}