Используйте FormData для отправки данных. И processData и contentType в false.
$('#NewsCreatorForm').submit(function() {
var formData = new FormData($(this)[0]);
$.ajax({
type: "POST",
url: "/engine/queryforms/postnews.php",
dataType: 'json',
data: formData,
processData: false,
contentType: false,
success: function(data)
{
if(data.result == 'success') {
$('#NewsCreatorSuccessModal').modal('show');
setTimeout(function(){document.location="/admin/news/"}, 1000);
} else {
for(var errorField in data.texterror){
document.getElementById('errortext').innerHTML = data.texterror[errorField];
$('#NewsCreatorErrorModal').modal('show');
}
}
}
});
return false;
});