I think it depends on what you want. There are lots of them already out there on the net, you just have to find them. If there's an image you want to use, there are plenty of programs, both free, shareware and paid, that will let you adjust the sizes. Even the paint program in Windows can do the really basic stuff.
If you want to create one from scratch, it gets more complicate, but can also be more fulfilling if you want to put some time into it. The built in Windows programs (assuming you actually use Windows) is the most basic, with others getting progressively more complex. Gimp is roughly equivalent to Photoshop, but also has a steep learning curve.
For drawing, you may want to look at
Inkscape, which is equivalent to Corel Draw or Illustrator. It also has a learning curve, but you should be able to do really fun stuff with it if you put in the time.
Hope this helps.