first you'll need a program to do it go there and download InfranView
http://www.irfanview.com/main_download_engl.htmrun the .exe be carefull not to instal ebay tool bar and those other crap you don't want them.
when it's on the file association thing, If you want InfranView to be your default picture viewer select all other then that continue to next step.(hit ok or next till the end)
By default it should be installed in c:\program files\infranView
Resizing the picture
go to the picture you want to resize right click on it then click on open with then search for infranView (c:\program files\infranView). Note if you choose to make infran view your default viewer just click on the image and it should open in it.
in InfranView hit the ctrl+R keys the resizing window will pop-up.
select set new size as a pourcentage of original and be sure that preserve aspect ratio is selected at the bottom.
resize to something like 70% then ok. If the pic is still too big hit ctrl+R and ...
when done go to file, save as, in the bottom rolldown menu select jpeg or jpg then click on save.(save it where you want it)