Use the scale option where iw is the input width and ih is the input height, to resize in half you can use the
division / operator and a colon as a separator between the width and height operations: iw/2:ih/2.
ffmpeg -i _MG_5010-2.jpg -vf scale="iw/2:ih/2" _MG_5010-2_small.jpg
To resize a list of images:
find . -name '*jpg' -exec ffmpeg -i {} -vf scale="iw/2:ih/2" {}.png \;
I this example I changed the filename extension to png for simplicity.