You've used php/lua/java settings to assign a value, but its bash.
index = $(printf %06d $(( $index + 1 )) ) --> index=$(printf %06d $(( $index + 1 )) )

And regarding the folders, i now assume it lookes like (yes the renaming of the folders is on purpose, just for the example):

Now according to your posted code snippet, i would change that to:
test.sh

Code:

#!/bin/bash
SOURCES="$HOME/Desktop/sourcepictures"
TARGET="$HOME/Desktop/orderedpictures"
LIST="$HOME/Desktop/list.txt"
index="0000000" 

for image in $(cat $LIST) ; do 
    ln -s $SOURCES/$image  $TARGET/$index.jpg
    index=$(printf %06d $(( $index + 1 )) )
done

If list.txt contains only the chars (a,b,c,d...) you need to add ".jpg" after "$image".
If list.txt contains the proper image name, you wont need to change anything.

hth & gn8