If you want to resize numerous images within a folder, you can use the ImageMagick command mogrify.
mogrify -resize 1200x -path ../smaller -format jpg *.JPG
The parameters are:
- resize: maximum with of 1200px
- path: where to store the resized images
- format: change the .JPG to .jpg
- *.JPG: resize all files within the folder with that naming