You can use paint to shrink it. Open paint then open the file from the paint menu. Go to image then stretch/skew. Keep the horizontal and vertical equal then start reducing the size of the pic. I would try like 15-20% to start with.
Make sure to have a virgin copy of the file saved so that if you F up, you can easily start over.
There are a billion ways to do this and a hundred programs to do this, but almost everyone has paint. You dont sound like you are running linux, so you should be set.
Be aware that after you shrink it the first time, then then next time you do it, you are reducing the already smaller picture size, so if you do 15% twice (example) then you are already down to 2% of the original size, which is tiny.
Unless you removed it, you have paint. You need to go to programs accessories, open paint. Then IN PAINT go to file then open. You have your computer set up to use microsoft picture to open all pics.
Or go to start, then run. In the run box type mspaint and hit enter. It should bring up the paint program.