I was under the impression that PNG/Gif/JPEG are already in compressed form. Your examples also do not show achieving any significant reduction in image size.
This has become more well-known than it has in the past, so perhaps there is less low-hanging fruit, but it's still a worthy part of your optimisation routine.
PNG and JPEG already compress the image, but the file format can be compressed still by removing extra data added by for example Photoshop.