If you just want to create a simple picture, Windows includes a program called Paint (located in programs -> accessories) which will suffice for this. It is rather limited though, but is probably a good place to get a feel for how to create images on a computer.
If you want to create animations, I have found Adobe Flash to be very versatile. With it you can create simple animations quite quickly and export them in a variety of web-friendly formats. Be aware it is rather expensive though.
I suggest giving Paint a try and if you like it do a search for free animation or free paint software on the internet. There are loads of free programs out there you can download and try out.
Good Luck.
