Incrementing a variable inside a for loop would probably be the easiest way, provided you don't have so many pictures that filename expansion causes a memory error.
For example:
Code:
index=1000 # Outside of quotes, preceding 0's disappear in variable values.
for picture in *.jpg; do
mv "${file}" pic-${index}.jpg
index=$((${index}+1)) # increment value.
done
Make a backup of this directory before you do this, because a simple typo could wipe out all of your pictures.
