Anything will do it, even paint.
In paint just open the jpeg, go to -> image -> stretch/scew
A window will pop up and the top part is the stretch, it will have 100% in both boxes just change them both to 50% and the size will shrink in half, both in actual size and viewing size.
If your file is a bmp or something open and save as a jpeg so it is much smaller.
Have fun.
Edit: dj types faster then me but I did it step by step so I win.
