*Updated 3/11/2012* with simpler method.
Compressing pictures/photos before sending to our friends is not necessary (because of large email storage and high speed broadband these days), but still a good way to show that you're considerate.
Here's a method so simple that anyone could do it. (Senior friendly)
1) Download and install Image Resizer for Windows.
2) In Windows Explorer, right click your picture and select 'Resize pictures'.
3) Select the size you want and click 'Resize' and you're done. Your compress picture file will be saved in the same folder.
PS: Normally I'll go with Medium compression which is good for everyday use. Be careful if you use 'Replace the originals' option because it will delete the original photo permanently.
To compress multiple pictures, hold down Ctrl key to select multiple pictures, right-click any of them and repeat the same steps!