for making gif thumbnails you need to have gd version which supports gif ... as the code will try to call functions specific to gif ... check your phpinfo() [under gd specifications] it will show you the list of mime types supported by GD installed with your php ...

for this array:

PHP Code:
$photo_types = array( 
'image/pjpeg' => 'jpg'
'image/jpeg' => 'jpg'
'image/gif' => 'gif'
'image/bmp' => 'bmp'
'image/x-png' => 'png' 
); 
you can simply remove the gif listing to disallow any gif uploads ...

PHP Code:
$photo_types = array( 
'image/pjpeg' => 'jpg'
'image/jpeg' => 'jpg'
'image/bmp' => 'bmp'
'image/x-png' => 'png' 
);