Check form fields: submit if not empty

<table CLASS="Admin">
	<form action="hideout/news/processNews.php?action=addNews" method="post" enctype="multipart/form-data" name="frmAddNews">
	<tr><TH id="header" COLSPAN="2">Nieuws toevoegen
	<tr> 
		<td width="150" >Titel
		<td><input name="txtTitle" type="text" id="input" size="50" maxlength="100">
	<tr> 
		<td width="150" >omschrijving
		<td> <textarea name="mtxMessage" cols="70" rows="10" id="input"></textarea>
	<tr> 
		<td width="150" >Afbeelding
		<td><input name="img" type="file" id="input"> 
	<TR>
		<TD COLSPAN='2' ALIGN='center'>
			<input name="btnAddNews" type="button" id="button" value="Toevoegen" onClick="checkAddNewsForm();">
			<input name="btnCancel" type="button" id="button" value="Terug" onClick="window.location.href='?action=hideout_news';"> 
	</TABLE>

function checkAddNewsForm(){
	with (window.document.frmAddNews){
		if (frmAddNews.txtTitle.value == "") {
			alert("Vul een titel in.");
			return;
		}
		if (frmAddNews.mtxMessage.value == "") {
			alert("Vul een omschrijving in.");
			return;
		}
		else {
			submit();
		}
	}
}