Mark - I use XnView m8. Been using it for years and its free. Here's the batch conversion screen....
Linky....
http://www.xnview.com/en/download.html
D.
You can do it in photoshop, under "scripts" which is in the file or edit menu. Then you just select the folder, tick the appropriate boxes and set the new sizes, photoshop runs through them all.
Simples