$image_info = getimagesize($chemin);
$image_type = $image_info[2];

if( $image_type == IMAGETYPE_JPEG ) {
$img_src_resource=imagecreatefromjpeg($chemin);
} elseif( $image_type == IMAGETYPE_GIF ) {
$img_src_resource=imagecreatefromgif($chemin);
} elseif( $image_type == IMAGETYPE_PNG ) {
$img_src_resource=imagecreatefrompng($chemin);
}

et faire de meme pour le reste, afin de gerer plus d extensions