The == is not working in my IF statement Function

i have a simple function (up) here its an arrow and onclick is should check for image 1 in the div "gallerymain" and if its there it should show image 2. ELSE if image 3 is displayed and it suppose to show image 4 .

however for some unknown reason the == isn't working (i checked with alert ) , i also made sure that all is in the right case and path . the funny thing is that if i will use = instead of == it will work but it onclick it will show image 2 no matter what (even if image 3 is there ) i understand why (because i am assigning the path to the src . maybe somebody please take a fresh look on it and see what am i missing.

thank you!

  function up () {         


     if 
        (document.getElementById("gallerymain").src == 
    "../images/gallery/image1.jpg") 
        {
        document.getElementById("gallerymain").src = 
    "../images/gallery/image2.jpg"
        }
    else if   
    (document.getElementById("gallerymain").src == 
    "../images/gallery/image3.jpg") 
        {
    document.getElementById("gallerymain").src = 
    "../images/gallery/image4.jpg"
        }
    }