What is the best program to use to create the graphics in android applications?

what is the best program to use to create the graphics in android applications? I'm developing an android game. I finished all the basic programming with pictures of dots instead of the real images. Now i want to make the game more graphically enticing. Is there any commonly used program that people use to create these images/sprites? I want to make the images look 3d but be 2d in reality. Basically making 2d images that create the 3d illusion effect.