Java 2D Graphics Issues

1) Is their performance loss when creating a new image with transparency transparency by painting just a few things on it and then painting on top of another; compared to the same image under the same conditions, but with the background cleared to an opaque color?

2) What is the importance of losing performance when drawing multiple images versus drawing one large image that includes all of them?

3) Is it better to draw small pieces of images from a large image (sprite sheet) or from their separate image? Note that I am talking about memory images, their representation on disk does not matter.