I always use paint since it is on pretty much any PC.
Here is a step by step process.
1) Right click the file
2) Click the "open with" option in the drop down menu
3) Select "paint" as the program to use
4) Once you are viewing the picture in paint click on the "image" menu
5) Click on the "stretch/skew" option
6) Put in percentages (i.e. 50% or 75%) until the picture is small enough. Just remember to put the same percentage in both the horizontal and vertical boxes so you don't distort your picture.
Hope that helps...