How to bulk rename files numerically, in order of modification date?

In the directory containing the pictures, run:

mkdir renamed; num=0; for f in $(ls -t); do cp -p "$f" renamed/IMG_$(printf "%03d" $num).jpg; num=$((num+1)); done

This will copy the files to a new directory renamed, keeping timestamps untouched. The format will be IMG_###.jpg with three-digit numbers, using leading zeros. You can change the number of digits by changing the %03d part.

Try PyRenamer. I have just installed it and it seems very powerful. It allows you to rename your images based on their metadata.