The Paint program comes with most computers. Go under the Start menu, then Programs, then Accessories. You should find the Paint program there.
Under Paint, choose Open, select the picture you want to change. Then pick the option Image which is along the top tool bar. You can then Decrease the Horizontal and Vertical size of the picture to whatever percentage you want. This will reduce the amount of bytes because you are adjusting the picture size.
This method allows you to keep the image quality on the high setting. Also before posting a pic, you can always check its size by right clicking on the picture and selecting Properties. This will show you the size (number of bytes)of the picture and whether or not you have to change it before posting here.
Hope this helps.