image_filter watermark /path/to/watermark.png [ bottom-right (default) | top-left | top-right | bottom-left | center ];
See also http://nginx.org/en/docs/http/ngx_http_image_filter_module.html#image_filter
For using in spec directly
...
%define with_watermark 1
...
%if 0%{?with_watermark}
# PATCH for watermarks
Patch7: nginx-watermark.patch
%endif
...
Tested with nginx 1.9.12