Next you'll optimize the logo and the sticker slices, and then optimize the remaining continuous-tone buttons in the JPEG and PNG-24 formats and compare these options.
To preserve the quality of continuous-tone and gradient-filled artwork, it's recommended that you use the JPEG image file format. Illustrator saves JPEG files using different compressions based on the specified image quality. The compression option you select determines how the color information in the artwork is preserved, which affects the size and quality of the image. It is important to keep in mind that JPEG doesn't support transparency.
Transparency makes it possible to place a nonrectangular graphic object against the background ...