All pictures can be processed by invoking script make_smaller.sh for each picture.
Example 2. Processing sequentially
#!/bin/bash
MAKE_SMALLER=$PWD/make_smaller.sh
IMGS_DIR=$PWD/imgs
TARGET_DIR=$PWD/imgs_smaller
for file in $IMGS_DIR/*; do
bash $MAKE_SMALLER $file > $TARGET_DIR/${file##*/}
done