Microsoft paint works, just open the image, go to image>sketch and skew, and type in a value less than 100 for both blanks. Usually I do 50 to scale the image to half the size, or even less.
That works unless it's a really large file because the site limits viewable size to 800 x 800 (I may be off a bit). That's why when I resize I choose to edit the actual pixels and make it 800 wide and let is scale the height properly (or vice cersa). This almost always brings the image under the file size limit of..150 kilobytes (I think again).