General :: Add (not Replacing) A Pattern Match With A Similar Pattern?

I'm writing a bash script to search html files, and when I find any occurrence of an img src tag like this:

<img src="123-picture-normal.jpg" alt="some random user entered text" border="0">

I want to add a second line below it that looks like:
<img src="123-picture-thumbnail.jpg" alt="some random user entered text" border="0">

All I need to do is duplicate the line but replace "normal" with "thumbnail"

Each file can have multiple img src tags with different numbered jpgs.

I have a feeling this is a job for sed, but I'm struggling with it. Any ideas?