I like it, thanks chromatic.   May I throw a slight curve in the fairway?
A quick dig in The Owl leads me to think something like this will match reference any image of alphanumeric name.   How close am I?

perl -i.bak -pe 's!(img  src=")(\w+?\.(gif|jpg|jpeg|png)")!$1/Images/$2!g' *.html

Update: The above one-liner was tweaked, and now works.

    Changes this:

<img src="12345.gif">
<img src="1b3e5.jpg">
<img src="1B3Df.jpeg">
<img src="abcde.png">
[download]

    To this:

<img src="/Images/12345.gif">
<img src="/Images/1b3e5.jpg">
<img src="/Images/1B3D5.jpeg">
<img src="/Images/abcde.png">
[download]

And saves original files as ".bak".
    cheers,
    Don
    striving for Perl Adept
    (it's pronounced "why-bick")