If you don't have a photo program on your computer, I would recommend irfanview. You can get it for free from
www.irfanview.com.
The resize feature is nice because if you are working with very large files (from a digital camera for instance) it can "resample" the picture as well as resize it.
This is good because just resizing a big picture can cause the smaller version to be very grainy.
Here's a good example of what I mean. This was done before I discovered irfanview.
This is actually one of the better ones. I went back and fixed all the bad ones...
The process in irfanview is similar to the process I outlined above for MS photo Editor.
Open up the picture in irfanview.
Click "image" in the top menu.
Then "Resize/Resample"
Same type of settings as mentioned above.
Just make sure the "Resample (higher quality) button is checked.
You can play with the different sampling settings if you like, but the default seems to work well..