Pixresizer is a great tool to:

First you need the tool.  Go to http://bluefive.pair.com/ and download Pixresizer.

Follow the instructions to install it.

 

The first choice is 1. One picture (tool calls it "one file") or 2. Multiple pictures.  That's selected with the tab at the top.

 

One picture (file)

1.  Click "load picture" that's like the "browse" function seen on other tools.

2.  Pick a size

3.  Select a format

4.  Click "save picture"

 

Multiple pictures at once.  Follow similar steps except you point to folders instead of a file.  It also means it will process ALL the files in the source folder so you may want to have a folder of just the pictures you want to process.