Add check that images were optimized

Add check whether an image was optimized. Just running simple ImageMagick can produce a huge difference:

```
magick original/esp-dongle-hardware-design.webp esp-dongle-hardware-design.webp
 288K 25 Oct 09:13 original/esp-dongle-hardware-design.webp
 52K 25 Oct 09:14 esp-dongle-hardware-design.webp
```