Div on Div with img element - html

Question

I want my Div id="b" to be on the other Div id="a", but instead I get the Div id="b" under Div id="a", What need I change in the Css?
Html:
<div class="picad30">
<div class="pic_sin30" id="a">
<img width="110" border="0" class="pic_sin30" src="SMALLAD.png"/>
</div>
<div class="picgar30 p" id="b"></div>
<div >
<div >upload</div>
<div ></div>
</div>
</div>
Css:
.picad30{
width:130px;
float:right;
display:table;
margin: 20px 0 0 0px;
position: relative;
}
.pic_sin30{
border: 1px solid #ccc;
height: 87px;
overflow: hidden;
width: 130px;
position: relative;
z-index:1;
}
.picgar30{
background-position:-5px -244px;
cursor:pointer;
height: 20px;
width: 20px;
float:left;
z-index:20;
position:absolute;
}
.p {
background-image: url("PIC/icon.png");
background-repeat: no-repeat;
}

Answer 1

Try this
.p {
background-image: url("PIC/icon.png");
background-repeat: no-repeat;
z-index:2;
position:absolute !important;
top:0;
left:0;
}
It can be helpful to you Read this
Edit
Working Fiddle

Answer 2

So I've made a few updates to your code and I hope I've interpreted your question correctly. Am I right by saying you want to stack .pic_gar30 or id=b on top of .pic_sin30 or id=a? And then on top of .pic_gar30 you want to add an icon?
If this is correct I've done the following, I've wrapped your 2 image divs in a parent div called pic-container which has the height you wish to use on your images and position relative so you can absolutely position your images inside without the upload text hiding in behind also.
You want to use the same css on both your image holders so I've given both of them the class img-holder and applied styles that position absolute, since .pic-gar30 comes after .pic_sin30 it will automatically be positioned on top. But if you like you can add a z-index to .pic_gar30. I've then used the ::after psuedo class selector http://css-tricks.com/pseudo-class-selectors/ on .pic_gar30 in order to position your icon above the image.
Please see my example which I hope helps http://jsfiddle.net/LXaCT/