added hint about image size

template/mumble/mumble.htm

@@ -44,6 +44,8 @@
 	    You can upload an image that you would like to use as your user texture here.<br />
 	    Your current texture is:<br />
 	    <img src="/mumble/{{DBaseObject.id}}/texture.jpg" alt="user texture" /><br />
+		Hint: The texture image <b>needs</b> to be 600x60 in size. If you upload an image with
+		a different size, it will be resized accordingly.<br />
 	  </p>
 	  <form action="" method="post" enctype="multipart/form-data">
 	    <table>