Well, there are many ways to create a picture, and upload it here. Some of the more obvious alternatives are:
(1) MS Paint, GIMP, or other paint programs
(2) Illustrator, Corel, Powerpoint, Inkscape, or other vector drawing packages
(3) TikZ or MetaPost or Asymptote, if you think that writing code is a good way to create pictures.
(4) Mathematica, Maple, Matlab, or any other package with graphing capabilities
(5) Draw on paper, and scan
(6) Draw on a whiteboard (or even a blackboard), and shoot with a camera or mobile phone.
Options 5 and 6 give you a bitmap image directly. The other options will require you to generate one indirectly. A screen grab is usually the easiest way.
For graphs, I would recommend #2 or #5, personally.