function IsNumeric(sText)
{
   var ValidChars = "0123456789";
   var IsNumber=true;
   var Char;
 
   for (i = 0; i < sText.length && IsNumber == true; i++) 
   { 
    Char = sText.charAt(i); 
    if (ValidChars.indexOf(Char) == -1) IsNumber = false;
   }
   
   return IsNumber;   
}
   
function checkform(form) 
{
    if (!IsNumeric(form.width.value) || form.width.value > 1600 || form.width.value < 1 ) 
	{
        alert("Error, width must be a number between 1 and 1600\n\n");
        return false;
    }
	
	if (!IsNumeric(form.height.value) || form.height.value > 1200 || form.height.value < 1 ) 
	{
        alert("Error, height must be a number between 1 and 1200\n\n");
        return false;
    }
	
	if (form.userfile.value == '') 
	{
        alert("Error, you must choose an image file from your PC to resize. Click the Browse button.\n\n");
        return false;
    }
		
	return true;
}