A brief breakdown:

#!/usr/bin/perl

@files=('img1.jpg','img2.jpg','img3.jpg');

$r = rand(4700)%(scalar(@files));

$file = $files[$r];

print "Content-type: image/";

print ((split(/\./,$file)[1]=='jpg')?'jpeg':'gif');

print "\n\n";

if (open(FID,$file)){
    while($data = <FID>){
        print $data;
    }
    close(FID);
}
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