Try this
.span3*3>img[src="/images/img_$.png"]
Or
.span3*3>img[src=/images/img_$.png][alt]
Output is
<div class="span3"><img src="/images/img_1.png" alt="" /></div>
<div class="span3"><img src="/images/img_2.png" alt="" /></div>
<div class="span3"><img src="/images/img_3.png" alt="" /></div>