Occasionally you may want to scan an image at a higher resolution to remove a moire pattern, or enlarge the image. When you are finished manipulating it, you must still set the dpi back to 72. Jpgs are always 24bit. See Optimizing Gifs . Graphic File Formats Different graphics file formats store data and compress it differently.
TIFF format is one of the largest because it saves an alpha channel in the file.
PICT files can be saved at any bit depth, and the file size will correspond accordingly. JPG and GIF files use special compression algorithms to reduce the file size of an image. Make sure you have your original somewhere and just make a copy to compress. Once you compress the data, the information is lost and you can't restore the image. I highly recommend that you control the process in an image editing program.