Change Picture Displayed with Mouse click

I am having troubles getting my code to change back and forth by clicking on a mouse button on an img.  Here is the javascript function:

[code]
function ShowOpenPOs(i, PartID)
{
    var url="PastDueOpenOrders-OpenPOs.asp?ID="+PartID
   
   
   
    if (document.images["imgPlusMinus"+i].src = "images/Plus.jpg")
    {
          alert(document.images["imgPlusMinus"+i].src)
        document.images["imgPlusMinus"+i].src = "images/Minus.jpg"
        alert(document.images["imgPlusMinus"+i].src)
    }else{
          alert("Minus")
        document.images["imgPlusMinus"+i].src = "images/Plus.jpg"
    }
   
      
      intI = i
    xmlHttp=GetXmlHttpObject(stateChanged)
    xmlHttp.open("GET", url , true)
    xmlHttp.send(null)
}
[/code]

Here is the img:

[code]
<img id="imgPlusMinus<%=i%>" alt="Show Open PO Details" src="images/test.jpg" onclick="ShowOpenPOs(<%=i%>, <%=rs1("ID")%>)">
[/code]

What happens from my troubleshooting is when it is a plus and then clicked, it goes to a minus...perfect.  Now, when I click it when it is a minus, it goes to a plus, then right back to a minus.  What am I doing wrong?