add segment before controller without changing base url - codeigniter

Question

I have a url like this
localhost/codeIgniter/Depan?lang=id
and I want to make it like this
localhost/codeIgniter/id/Depan
where localhost/codeIgniter is the base_url, /id is parameter for language, and /Depan is the controller name. But since I change the url as the second one. The base_url is changed to localhost/codeIgniter/id. So everytime I pass images location like assets/image/pic1.jpg from javascript. The link in html become localhost/codeIgniter/id/assets/image/pic1.jpg. And the file that actually stored in codeIgniter/assets/images folder is can't accessed. Ive already set the
$config['base_url'] = 'localhost/codeIgniter'
I wish I could use the second url without changing the base url
HTML
<div class="picture container">
<img id="slide" src="">
<div class="desc">
<div id="text">
<p></p>
</div>
<div id="text">
</div>
</div>
<div onclick="prev()" class="prev">
<img src="<?php echo base_url();?>assets/button/prev.png">
</div>
<div onclick="next()" class="next">
<img src="<?php echo base_url();?>assets/button/next.png">
</div>
<div class="pics">
</div>
<script> var sldrpic = <?php echo json_encode($sldrpic); ?>;</script>
<script src="<?php echo base_url();?>javascript/sldr_home.js"></script>
</div>
Javascript
function firstslide() {
if (!document.images)
return
document.getElementById('slide').src = slideimages[step].src;
document.getElementById('slide').alt = sldrpic[step].nama;
document.getElementById('text').childNodes[1].innerHTML = sldrpic[step].nama;
}
with the javascript I want to pass the slideimages[] array that contain folder path assets/image/pic.jpg to the
<img id="slide" src="">

Answer 1

I have put base url variable in html as shown below.
<div class="picture container">
<img id="slide" src="">
<div class="desc">
<div id="text">
<p></p>
</div>
<div id="text">
</div>
</div>
<div onclick="prev()" class="prev">
<img src="<?php echo base_url();?>assets/button/prev.png">
</div>
<div onclick="next()" class="next">
<img src="<?php echo base_url();?>assets/button/next.png">
</div>
<div class="pics">
</div>
<script> var sldrpic = <?php echo json_encode($sldrpic); ?>;</script>
// echo base url from php
<script> var base_url ="<?php echo base_url();?>"; </script>
<script src="<?php echo base_url();?>javascript/sldr_home.js"></script>
</div>
javascript
function firstslide() {
if (!document.images)
return
// note: base_url is variable. not function.
document.getElementById('slide').src = base_url + slideimages[step].src;
document.getElementById('slide').alt = sldrpic[step].nama;
document.getElementById('text').childNodes[1].innerHTML = sldrpic[step].nama;
}