Creating sigs is not as easy as it looks. Sigs aren't mere cropped versions of stolen images, but rather an intricate work of art. A new sig typically begins with what we call "renders". Renders are images with transparent or neutral backgrounds, usually with a subject/s cleanly cut around its borders. But of course, you can always start with a full image instead.
Some excellent sources of renders can be found at:
The render is exported to the canvass wherein the sig makers begins work. Now, there are several graphics manipulation programs to choose from. Among the most popular are:
Adobe Photoshop is my software of choice, just because I've been using it for a long time and for me, it's the best one out there. However, Fireworks and CorelDRAW are good alternatives. GIMP also does a great job and it's free. If you're looking for tutorials, check out Pixel2Life and Hybrid Genesis. The latter provides free brushes and textures as well.