using egrep to rearrange file names

I'm sure this is an easy one but I can't seem to get it working. Given the following:

for oldName in `ls *.JPG` ;do
newName=<confusion here. how to make sed perform 's/.JPG/_thumb.JPG/g' operation on $oldName>
done

Could someone show me how to make this work?

Thanks

Ken

Well that was fun...

Here's the solution I came up with to duplicate a group of files, selected by pattern into the current working directory. Hope this saves someone else some time.

#!/bin/sh

# change values of following two variables to whatever you want
oldPattern=".JPG"
newPattern="_copy.JPG"

for tmp in `ls *.$oldPattern | sed "s/$oldPattern//g"`; do
# create the new file
touch $tmp$newPattern

# copy contents of old file into new file
cp $tmp$oldPattern $tmp$newPattern
done

Ken