goto:
http://www.irfanview.com and download install it. Then click on your image and inside irfanview CLICK
Image then
Resize/resample to choose your new size then Save it. If you are referring to shrinking the viewing size this awesome freeware will do it. If you are referring to the file size (bytes) there are file shrinkers online which can reduce the bytes to allow quicker loading times with a small sacrifice in quality.
