MS Paint, comes with every single version of Windows
If you don't know where it is on your computer, you can just go to the Windows menu on the bottom left corner of your screen, click on that. In the blank search window that pops up immediately above the Windows symbol, you can just type "mspaint" (no quotes) and hit enter. That will bring up the program.
Open your image, then you should be able to simply click "resize" which is in the upper graphic menu bar, change the view to "pixels" and use a number that is 1/2 of what you are currently at. This should be a sufficient resize. Then file-->save and you are done.