For convert image to format webp you need use webp utility. She support next input formats: WebP, JPEG, PNG, PNM (PGM, PPM, PAM), TIFF

  1. Install webp on macOS:
brew install webp
  1. Run command:
cwebp image.jpg -o image.webp

Output:

Saving file './image.webp'
File:      ./image.jpg
Dimension: 500 x 500
Output:    4212 bytes Y-U-V-All-PSNR 42.56 48.59 47.40   43.74 dB
           (0.13 bpp)
block count:  intra4:        248  (24.22%)
              intra16:       776  (75.78%)
              skipped:       591  (57.71%)
bytes used:  header:             39  (0.9%)
             mode-partition:   1279  (30.4%)
 Residuals bytes  |segment 1|segment 2|segment 3|segment 4|  total
    macroblocks:  |       2%|       7%|      13%|      77%|    1024
      quantizer:  |      52 |      52 |      46 |      35 |
   filter level:  |      16 |      12 |      48 |      37 |

Use -q for chnage quality. Where quality is between 0 (poor) to 100 (very good). Typical value is around 80.

For convert more images right away use cycle for:

for i in path_to_images_dir/*; do cwebp -q 50 "$i" -o "${i%.*}.webp"; done