3 added 4 characters in body
source | link

PHP, 192194 bytes

My previous answer did not animate; this one does.

<?session_register(i"i");$i||imagecreatetruecolor(64,64);imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24);header("Content-Type: image/png");header("Location: ".basename(__FILE__));imagepng($i);

save to file, call in browser. It´s as fast as your browser can refresh.
Probably not able to set 4096 pixels in 180 seconds though.

breakdown

<?                                                  # start PHP
session_register(i"i");                                # register image in session
$i||imagecreatetruecolor(64,64);                    # if no image exists, create one
imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24); # set a random pixel to random color
header("Content-Type: image/png");                  # send HTTP header for image 
header("Location: ".basename(__FILE__));            # send HTTP header to reload
imagepng($i);                                       # send image data

PHP, 192 bytes

My previous answer did not animate; this one does.

<?session_register(i);$i||imagecreatetruecolor(64,64);imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24);header("Content-Type: image/png");header("Location: ".basename(__FILE__));imagepng($i);

save to file, call in browser. It´s as fast as your browser can refresh.
Probably not able to set 4096 pixels in 180 seconds though.

breakdown

<?                                                  # start PHP
session_register(i);                                # register image in session
$i||imagecreatetruecolor(64,64);                    # if no image exists, create one
imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24); # set a random pixel to random color
header("Content-Type: image/png");                  # send HTTP header for image 
header("Location: ".basename(__FILE__));            # send HTTP header to reload
imagepng($i);                                       # send image data

PHP, 194 bytes

My previous answer did not animate; this one does.

<?session_register("i");$i||imagecreatetruecolor(64,64);imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24);header("Content-Type: image/png");header("Location: ".basename(__FILE__));imagepng($i);

save to file, call in browser. It´s as fast as your browser can refresh.
Probably not able to set 4096 pixels in 180 seconds though.

breakdown

<?                                                  # start PHP
session_register("i");                              # register image in session
$i||imagecreatetruecolor(64,64);                    # if no image exists, create one
imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24); # set a random pixel to random color
header("Content-Type: image/png");                  # send HTTP header for image 
header("Location: ".basename(__FILE__));            # send HTTP header to reload
imagepng($i);                                       # send image data
2 made an animated version
source | link

PHP, 103192 bytes

My previous answer did not animate; this one does.

$i=imagecreatetruecolor<?session_register(40,40i);for;$i||imagecreatetruecolor($p=1600;$p--;64,64)imagesetpixel;imagesetpixel($i,$p%40,$prand(64)%,rand(1)%64,1<<24rand()%2**24);header("Content-Type: image/png");header("Location: ".basename(__FILE__));imagepng($i);

PNG data is sentsave to STDOUTfile, call in browser. It´s as fast as your browser can refresh.
To display in browser, prepend <?header("Content-Type: image/png"); and saveProbably not able to fileset 4096 pixels in 180 seconds though.


 

1<<24 is #000000 with opacity 1 (which makes no difference for black pixels).breakdown

<?                                                  # start PHP
session_register(i);                                # register image in session
$i||imagecreatetruecolor(64,64);                    # if no image exists, create one
imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24); # set a random pixel to random color
header("Content-Type: image/png");                  # send HTTP header for image 
header("Location: ".basename(__FILE__));            # send HTTP header to reload
imagepng($i);                                       # send image data

PHP, 103 bytes

$i=imagecreatetruecolor(40,40);for($p=1600;$p--;)imagesetpixel($i,$p%40,$p,rand(1,1<<24));imagepng($i);

PNG data is sent to STDOUT.
To display in browser, prepend <?header("Content-Type: image/png"); and save to file.


 

1<<24 is #000000 with opacity 1 (which makes no difference for black pixels).

PHP, 192 bytes

My previous answer did not animate; this one does.

<?session_register(i);$i||imagecreatetruecolor(64,64);imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24);header("Content-Type: image/png");header("Location: ".basename(__FILE__));imagepng($i);

save to file, call in browser. It´s as fast as your browser can refresh.
Probably not able to set 4096 pixels in 180 seconds though.

breakdown

<?                                                  # start PHP
session_register(i);                                # register image in session
$i||imagecreatetruecolor(64,64);                    # if no image exists, create one
imagesetpixel($i,rand(64)%,rand()%64,rand()%2**24); # set a random pixel to random color
header("Content-Type: image/png");                  # send HTTP header for image 
header("Location: ".basename(__FILE__));            # send HTTP header to reload
imagepng($i);                                       # send image data
1
source | link

PHP, 103 bytes

$i=imagecreatetruecolor(40,40);for($p=1600;$p--;)imagesetpixel($i,$p%40,$p,rand(1,1<<24));imagepng($i);

PNG data is sent to STDOUT.
To display in browser, prepend <?header("Content-Type: image/png"); and save to file.


1<<24 is #000000 with opacity 1 (which makes no difference for black pixels).