imagejpeg() creates a JPEG file from
   the given image.
  
imageAn image resource, returned by one of the image creation functions, such as imagecreatetruecolor().
filenameThe path to the saved file. If not set or NULL, the raw image stream will be outputed directly.
       To skip this argument in order to provide the 
       quality parameter, use NULL.
      
quality
       quality is optional, and ranges from 0 (worst
       quality, smaller file) to 100 (best quality, biggest file). The 
       default is the default IJG quality value (about 75).
      
Note: JPEG support is only available if PHP was compiled against GD-1.8 or later.
Note: If you want to output Progressive JPEGs, you need to set interlacing on with imageinterlace().