Photoshop Elements
I don't use any Microsoft software, so I can't tell you how to use it.
If you want a very good and cheap digital photo editor, check out Adobe Photoshop Elements (about $79). This has a lot of the features the full version of Adobe Photoshop has but its 1/5 the price. It's meant for the digital photographer. If you do any web work, set your preferences to pixels instead of inches. Inches does not mean anything to a website, which measure everything in Pixels plus it does not care what DPI setting you use (dots per inch is only used for printing).
140x100 pixels works well for your avatar.