Why can't the submit button be an image?

// JavaScript Document
var err;
function CalcThis() {
 
	err = "";
	var taxRate = document.frmStockCalculation.slcTaxRate.value;
	var marketValue = document.frmStockCalculation.txtMarketValue.value;
	var originalCost = document.frmStockCalculation.txtOriginalCost.value;
	
	//Check empty or not number fields
	if (!taxRate) {err = "Please select your marginal tax rate.";}
	numCheck(marketValue,"current market value of your stock");
	numCheck(originalCost,"original cost of the stock");
	
	if(err) {
		alert(err);
	} else {
		taxRate = parseInt(taxRate);
		marketValue = parseInt(marketValue);
		originalCost = parseInt(originalCost);
		
		var taxCredit = marketValue * taxRate * 0.01;
		var capitalGain = (marketValue - originalCost) * 0.5 * taxRate * 0.01;
		var donation_direct = marketValue - taxCredit;
		var donation_sell = marketValue - taxCredit + capitalGain;
		
		//1. Amount of your donation
		document.getElementById("lblAmount_Direct").innerHTML = formatCurrency(marketValue);
		document.getElementById("lblAmount_Sell").innerHTML = formatCurrency(marketValue);
		
		//2. Less your tax credit
		document.getElementById("lblTaxCredit_Direct").innerHTML = formatCurrency(taxCredit * (-1));
		document.getElementById("lblTaxCredit_Sell").innerHTML = formatCurrency(taxCredit * (-1));
		
		//3. Plus tax on the capital gain
		document.getElementById("lblCapitalGain_Direct").innerHTML = formatCurrency(0);
		document.getElementById("lblCapitalGain_Sell").innerHTML = formatCurrency(capitalGain);
		
		//4. After tax cost of your donation
		document.getElementById("lblAfterTaxCost_Direct").innerHTML = formatCurrency(donation_direct);
		document.getElementById("lblAfterTaxCost_Sell").innerHTML = formatCurrency(donation_sell);
		
		//5. Net Benefit
		document.getElementById("lblNetBenefit_Direct").innerHTML = formatCurrency(donation_sell - donation_direct);
	}
}
function numCheck(num,att) {
	
	if (!err) {
		if (!num) {err = "Please enter "+att+".";}
		else if (isNaN(num)) {err = "Please enter number for "+att+".";}
	}
}
function formatCurrency(num) {
	
	var sign, cents;
	num = num.toString().replace(/\$|\,/g,'');
	if(isNaN(num)) num = "0";
	sign = (num == (num = Math.abs(num)));
	num = Math.floor(num*100+0.50000000001);
	cents = num%100;
	num = Math.round(num/100).toString();
	//num = Math.floor(num/100).toString();
	if(cents<10)cents = "0" + cents;
	for (var i = 0; i < Math.floor((num.length-(1+i))/3); i++)
	num = num.substring(0,num.length-(4*i+3))+','+
	num.substring(num.length-(4*i+3));
	return (((sign)?'':'-') + '$' + num);
	//return (((sign)?'':'-') + '$' + num + '.' + cents);
}