Biblioteca GD

Consigo fazer upload de algumas imagens em JPEG ultilizando a Biblioteca GD, mas algumas mostram esse erro:

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<b>Warning</b>: imagecreatetruecolor(): Invalid image dimensions in <b>/var/www/html/usuarios/insert.php</b> on line <b>112</b><br />

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<b>Warning</b>: imagecopyresized(): supplied argument is not a valid Image resource in <b>/var/www/html/usuarios/insert.php</b> on line <b>115</b><br />

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<b>Warning</b>: imagejpeg(): supplied argument is not a valid Image resource in <b>/var/www/html/usuarios/insert.php</b> on line <b>118</b><br />

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<b>Warning</b>: Cannot modify header information - headers already sent by (output started at /var/www/html/usuarios/insert.php:112) in <b>/var/www/html/usuarios/insert.php</b> on line <b>129</b><br />

O que significa?