http://www.irfanview.com/
Brilliant all-round image viewer, but it has features for resizing photos etc (can even batch resize, it's what I use to resize all my pictures from breach before uploading them - camera produces 5Mb+ photos)
To re-size, open the image, then go to Image > Resize/Resample
You will be able to resize the image there, then save it.
Hope that helps