[DELETED]

go to css file

search
header or logo and past your image url thats all

OK here's one way to do it:
go and upload your logo to wp-content/themes/(your theme)/images/

i wrote the code assuming your filename is logo.png. You can change that yourself.

Remove this:
<?php bloginfo('name'); ?> and this <span><?php bloginfo('description'); ?></span>

Be careful not to messed up with the <a href=""></a> anchor text,

Go to: style.css
insert this:

Example: Logo name: logo.png, height: 150px, and width: 350px

.newlogo {
background: url('logo.png');
height: 150px;
width: 350px;
}

Then, instead of:

OLD CODE:
<div id="headerleft">
<h1>
<a href="<?php bloginfo('url'); ?>" title="<?php bloginfo('description'); ?>">
<?php bloginfo('name'); ?></a><span><?php bloginfo('description'); ?></span>
</h1>
</div>

NEW CODE:
<div id="headerleft">
<h1>
<a href="<?php bloginfo('url'); ?>" title="<?php bloginfo('description'); ?>">
</a>
</h1>
</div>

Just like your css style is, but be sure about the file url of the image!

Luck!

Double Posting, so i deleted this one!