A ImageMagick tool

Resize the image

• convert -resize 320x240 FROM_IMAGE_PATH.(png/svg) FINAL_IMAGE_PATH.(png/svg)
• convert -resize 50% FROM_IMAGE_PATH.(png/svg) FINAL_IMAGE_PATH.(png/svg)

Pixilate Image

• convert -scale 5% -scale 1500% SOURCE.jpg pixelated.jpg
  • change the scales as required

References

1. command line - How to resize an image through the terminal? - Ask Ubuntu

2. software recommendation - How can I pixelate an image via the command line on Linux - Graphic Design Stack Exchange