I am trying to bring an image down without using the <br> tag but it is not working

I am trying to bring an image down lower in a subsection but apparently it is not working.  I have a <div> followed by a sub <div> which is supposed to displace an image but it is not budging.  Any idea why?  I will show the code here of my CSS. and HTML code.  Thanks for your help ahead of time.

Here is the code that I am dealing with in the HTML file:

<div id="bottomHeader">                                    
                <div id="submenus">                                                        
                    <div id="submenu1">                    
                        <asp:HyperLink ID="submenuhyperlink1" ImageUrl="~/images/submenu1.jpg" NavigateUrl="" runat="server"></asp:HyperLink>
                    </div>                  
                </div>            
        </div>      

and here is the CSS I am dealing with in another file:

#bottomHeader {
text-align:center;
background-image:url('../images/prggroupsite_bottomheader.jpg');
height:150px;
}

#submenus {
height:105px;
width:990px;
border-color:Black;
margin-left:100px;
margin-top:10px;

}

#submenu1 {
height:98px;
width:318px;
margin-top:10px;

}

Why is the image not being displaced exactly where I want it to be?  Any solutions to this problem?  Please let me know.  Thanks!