Open your picture up in Microsoft Paint(right-click picture, scroll-down to "Open With"; go to "Paint"):

1) open the "Image" tab, scroll-down to "Stretch/Skew"
2) reset horizontal size to 50%
3) reset vertical size to 50%
4) click OK
5) repeat above procedures(#1-4, for sufficient size reduction. 100% vertical/horizontal size reset will result in NO picture!)
6) open "File" tab
7) scroll-down to "Save As"
8) rename your newly-reduced picture
9) upload/post picture