The html for my tabs is as follows:

HTML Code:
<!-- Tabs -->
			<div id="tabs">
				<div id="tabs-line">
					<div class="tabs-back container_16"> 
						<div class="tabs-wrap clearfix">
							<div class="tabs-nav grid_16">
								<ul>
									<li class="aud"><a href="#audience"> Audience </a></li>
									<li class="opp"><a href="#opportunities">Opportunities</a></li>
									<li class="key"><a href="#keytopics">Key Topics</a></li>
									<li class="res"><a href="#research">Research</a></li>	
								</ul>
							</div>
						</div>	

						<div class="tabs_cont_wrap clearfix">
							<div id="audience">
							
									<div class="aud_pic grid_8">
									</div>
									
									<div class="aud_txt grid_8"> 
										<h2>Who Should Attend?</h2>
										<ul> 
											<li>C-level business development professionals</li>
											<li>Marketing & sales executives</li>
											<li>Others seeking effective strategies for marketing<br>
											health products & services to and through health <br>
											practitioners.</li>
										</ul>
									</div>		
							</div>
						
						
							<div id="opportunities">
									<div class="opp1 grid_16">
										<h2>The Benefits of Attending</h2>
									</div>
									<div class="opp2 grid_8">
										<ul>
											<li>Get an edge on competitors with the first presentation of
											data from HPC's 2013 Physician Survey</li>
											<li>Learn from nationally recognized experts in practitioner
											marketing & sales</li>
											<li>Explore real-world practitioner channel success stories</li>
										</ul>	
									</div>	
									<div class="opp3 grid_8">	
										<ul>
											<li>Learn strategies from the pharma & medical foods playbook</li>
											<li>Understand unique legal and regulatory challenges</li>
											<li>Hear clinicians describe what really 'speaks' to them</li>
											<li>Network with potential channel partners</li>
										</ul>
									</div>
							</div>
							
							
							<div id="keytopics">
								<div class="key grid_16">
									<h2>Key Topics</h2>
								</div>	
								<div class="key1 grid_4">
									<ul>
										<li>Building your brand by building trust</li>
										<li>Understanding the dynamics of today's healthcare</li>
									</ul>
								</div>	
								<div class="key2 grid_4">
									<ul>	
										<li>Communicating efficacy & safety to diverse practitioner segments</li>
										<li>Identifying unmet needs & assessing channel opportunities</li>
									</ul>
								</div>	
								<div class="key3 grid_4">
									<ul>
										<li>Navigating claims & other regulations</li>
										<li>Adapting the pharma & medical foods playbook for supplement industry metrics</li>
									</ul>	
								</div>
								<div class="key4 grid_4">
									<ul>
										<li>Strategies for market entry & brand development</li>
										<li>Effective selling through medical conferences, professional publications, direct mail, and office visit</li>
									</ul>
								</div>				
							</div>
							
							
							<div id="research">
								<div class="res grid_16">
									<h2>The Practitioner Channel is Booming! Our research shows... </h2>
								</div>
								<div class= "res1 grid_4">
									<p> The Practitioner Channel 
									presents unique business opportunities 
									AND unique marketing challenges that 
									require sophisticated 
									multi-tiered strategies & 
									long-term brand 
									development.</p>
								</div>	
								<div class="res2 grid_4">	
									<h5>80%</h5> 
										<p class="subtxt">of primary care doctors now recommend supplements</p>
										<p><span class="ref">(Holistic Primary Care 2010 Physician Survey)</span></p>
								</div>		
								<div class="res3 grid_4">
									<h5>13% </h5>
									<p class="subtxt">currently dispense supplements... and the numbers are growing</p> 
									<p><span class="ref">(Holistic Primary Care 2010 Physician Survey)</span></p>
								</div>
								<div class="res4 grid_4">				
									<h5>$2.8<span class="subtxt">billion</span></h5>
									<p class="subtxt">currently dispense supplements... and the numbers are growing </p>
									<p><span class="ref">(Holistic Primary Care 2010 Physician Survey)</span></p>
								</div>
							</div>
							
						</div>
					</div>
				</div>	
			</div>
Here is the css that pertains to how the tabs look:

Code:
li.aud  a{
	background: url(../images/tabs/aud-grey.png) no-repeat;
}
li.aud  a:hover{
	background: url(../images/tabs/aud-grey-hover.png) no-repeat;
}

li.opp a{	
	background: url(../images/tabs/opp-grey.png) no-repeat;
}
li.opp a:hover{	
	background: url(../images/tabs/opp-grey-hover.png) no-repeat;
}
li.key a{
	background: url(../images/tabs/key-grey.png) no-repeat;
}
li.key a:hover{
	background: url(../images/tabs/key-grey-hover.png) no-repeat;
}
li.res a{
	background: url(../images/tabs/res-grey.png) no-repeat;
}
li.res a:hover{
	background: url(../images/tabs/res-grey-hover.png) no-repeat;
}
So basically right now, the look of the tab changes when I hover over it. I would like to make it so that the tab would change to a third image when it is selected and active. I'm not sure what the term for this is since "active" implies just the time period that the item is being clicked. Please help!