I assume you don't have photo editing software. If you run windows as your OS try this.
Right click on the picture and then scroll down and choose "open with"; choose "paint"; Once the picture opens in windows paint, click on "image", and then click on "stretch/skew" ; you'll see horizontal and vertical set at 100%; if you want the picture half-sized, change both to 50%; 1/4 size is 25% and so on. Then go back up to the top, click on file, choose "save as", give the new-sized image a name, and decide where you want to save it. That's it!