The simplest way I found to watermark my timelapse images is with montage.

I'm using this in my raspberry-pi-timelapse:

DATE=$(date -u +"%Y-%m-%dT%H:%M:%SZ")
PATH_TO_IMAGE=example.jpg

montage \
  -label "$DATE" \
  $PATH_TO_IMAGE \
  -pointsize 30 \
  -gravity Center \
  -geometry +0+0 \
  $PATH_TO_IMAGE

This will add a simple watermark (with white background) at the bottom of the image, like this:

watermarked-example.jpg