I started with the lower right corner, and need to drag to the upper left corner. But the upper left corner is above the image of the toolbox window. I can now continue to drag up, and as long as I keep the triangle in the same place, the left side will be perfect when I finally get the top side to the proper place. Note this works best at high magnifications, because otherwise it's hard to know if the triangle is positioned right. Because of the compressive nature of .jpg files, color reduction does little to reduce the size of .jpg files.
Color reduction is most effective reducing the size of .png and .gif files.
Note that because .gif files are covered by a patent, you should not create new .gif files, and should take steps to convert your old .gif to .png. But anyway, here's a script, called 256, which reduces the depth of a .png to 256 colors. The reason we convert down gradually is to get the best color simulation. Also, always crop before you color reduce, so that extraneous colors that got cropped away don't influence the color approximations. The next article explains how to resize selections.