There are 2 different ways to change the number of bytes an image file takes uses.

1) Increase or decrease the actual size of the image, for example, shrink a 1024x768 pixel file to 800x600. This will make the image look either larger or smaller on the screen.
2) Increase or decrease the compression ratio. The image will look the same size, but the number of bytes in the file will increase or decrease.

Greg