Assuming you're talking about IOS here,
The two ways I know doing this is basically, in XCODE, making a tabbed application, going to he Storyboard file, connecting the tab views to the controller. Then make images for the tabs and replace them.
That, as I found was replicated here: http://kurrytran.blogspot.tw/2011/10/ios-5-tutorial-creating-custom-tab-bar.html
In Html5, I think you can do this:
<nav id="tabbar">
<ul>
<!-- Action & navigation -->
<li style="width: 20%; "><a href="#home" class="flip" style=""><strong>Home</strong><div class="home"></div></a></li>
<li style="width: 20%; "><a href="#cat1" class="flip current" style=""><strong>RSS Feed</strong><div class="rss"></div></a></li>
<li style="width: 20%; "><a href="#cat2" class="flip" style=""> <strong>Twitter</strong><div class="twitter"></div></a></li>
<li style="width: 20%; "><a href="#cat3" class="flip" style=""><small class="badge right">3</small><strong>Flickr</strong><div class="flickr"></div></a></li>
<li style="width: 20%; "><a onclick="moveDown();" class="flip" style=""><strong>Search</strong><div class="search2"></div></a></li>
</ul>
</nav>
EDIT:
Also, one way of making it fit is to resize the images so their is blank space on the sides, and change images for iphone5-portrait, ipad2-landscape, etc.
EDIT2:
Other than that , the only other way is to use UIToolbar to create a tabbar like creation with custom defined button presets. Then change views.
If not, then it is impossible or switch for a html element like the one above.