[demonfatal]

Just replace in the header template:

<fieldset id="logindetails" class="logindetails">
<div>
<div>
<input type="text" class="textbox" name="vb_login_username" id="navbar_username" size="10" accesskey="u" tabindex="101" value="<vb:if condition="$username">{vb:raw username}<vb:else />{vb:rawphrase username}</vb:if>" />
<input type="text" class="textbox default-value" tabindex="102" name="vb_login_password_hint" id="navbar_password_hint" size="10" value="{vb:rawphrase password}" style="display:none;" />
<input type="password" class="textbox" tabindex="102" name="vb_login_password" id="navbar_password" size="10" />
<input type="submit" class="loginbutton" tabindex="104" value="{vb:rawphrase log_in}" title="{vb:rawphrase enter_username_to_login_or_register}" accesskey="s" />
</div>
</div>
</fieldset>
<div id="remember" class="remember">
<label for="cb_cookieuser_navbar"><input type="checkbox" name="cookieuser" value="1" id="cb_cookieuser_navbar" class="cb_cookieuser_navbar" accesskey="c" tabindex="103" /> {vb:rawphrase remember_me}</label>
</div>

With that:

<fieldset id="logindetails" class="logindetails">
<div>
<div>
<style type="text/css">
.toplinks form img {
        position:relative;  
        top: 4px;        
} 
</style>
<img src="images/misc/username.png" alt="Username" /> 
<input type="text" class="textbox" name="vb_login_username" id="navbar_username" size="10" accesskey="u" tabindex="101" value="<vb:if condition="$username">{vb:raw username}<vb:else />{vb:rawphrase username}</vb:if>" />
<img src="images/misc/password.png" alt="Password" /> 
<input type="text" class="textbox default-value" tabindex="102" name="vb_login_password_hint" id="navbar_password_hint" size="10" value="{vb:rawphrase password}" style="display:none;" />
<label for="cb_cookieuser_navbar">
<input type="checkbox" name="cookieuser" type="hidden" value="1" tabindex="103" id="cb_cookieuser_navbar" class="cb_cookieuser_navbar" accesskey="c" checked="checked" /> </label> 

<input type="password" class="textbox" tabindex="102" name="vb_login_password" id="navbar_password" size="10" />
<input type="submit" class="loginbutton" tabindex="104" value="{vb:rawphrase log_in}" title="{vb:rawphrase enter_username_to_login_or_register}" accesskey="s" />
</div>
</div>
</fieldset>

Where the PNG images are located in the folder "images/misc/"