This PHP function uses the GD library to invert color or black and white images.
You can choose wether you want to save the negative of the image or just display it.
choose color or black and white output - works with .jpg and .png images if your GD-Library supports it.

“invert image” makes use of the built-in PHP functions imagecreatefromjpeg( ), imagecreatefrompng( ), imagesx( ), imagesy( ), imagecolorat( ), imagecolorsforindex( ), imagecolorresolve( ), imagecolorclosesthwb( ), imagesetpixel( ), header( ), imagejpeg( ) and imagepng( ).

01 <?php
02 
03 function invert_image($input,$output,$color=false,$type='jpeg')
04 {
05     if($type == 'jpeg') $bild = imagecreatefromjpeg($input);
06     else $bild = imagecreatefrompng($input);
07  
08     $x = imagesx($bild);
09     $y = imagesy($bild);
10  
11     for($i=0; $i<$y; $i++)
12     {
13         for($j=0; $j<$x; $j++)
14         {
15             $pos = imagecolorat($bild, $j, $i);
16             $f = imagecolorsforindex($bild, $pos);
17             if($color == true)
18             {
19                 $col = imagecolorresolve($bild, 255-$f['red'], 255-$f['green'], 255-$f['blue']);
20             }else{
21                 $gst = $f['red']*0.15 + $f['green']*0.5 + $f['blue']*0.35;
22                 $col = imagecolorclosesthwb($bild, 255-$gst, 255-$gst, 255-$gst);
23             }
24             imagesetpixel($bild, $j, $i, $col);
25         }
26     }
27     if(empty($output)) header('Content-type: image/'.$type);
28     if($type == 'jpeg') imagejpeg($bild,$output,90);
29     else imagepng($bild,$output);
30 }
31  
32 $input = 'path/to/image.jpg';
33  
34 // for a color negative image, set the optional flag
35 invert_image($input,'',true);
36  
37 // for a black and withe negative image use like this
38 //
39 // invert_image($input,'');
40 
41 // if you want to save the output instead of just showing it,
42 // set the output to the path where you want to save the inverted image
43 //
44 // invert_image('path/to/original/image.jpg','path/to/save/inverted-image.jpg');
45 
46 // if you want to use png you have to set the color flag as
47 // true or false and define the imagetype in the function call
48 //
49 // invert_image('path/to/image.png','',false,'png');
50 
51  
52 ?>