Gadget de mudar background

Resultado 1

Resultado 2

1. Vá ao painel do blog > Layout > Adicionar gadget
2. Escolha a opção HTML/JavaScript
Para o resultado 1, cole:

<div style="text-align:center;"></div><br /><center>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#ff0000'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYKefBNC19nRKyUz0VNSvoqTgdZLXsaGkM4DDmJuAd1RDYtmUv7_Ap6siEGPffbGtetRsairElJo3UXv5-35r3ovfkXqR4JNcdbDqzlFnuasT-eapiQlpdlvGgUcND7Xt6sY85K0PHO80r/s200/body-rojo.png" /></a>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#fff000'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgn5gVwmdTpMcV7kEunrUES-fTH100r_1iG4uCvvyxTEKH-YoSXk0YWTJf_h8og9jfInskULcdoA7iyxOz_G4v4Qoa0CZ26Bk_KQ7sVlSYMVr2kuw8M5RXT1_IhyphenhyphenXH6YyFM99-fADAZLs_G/s200/body-amarillo.png" /></a>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#04B404'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjyPY3k7KJwaYdV0qQPCBJAtSOa0qNA8P3AUHd8OfnK3EN5x0gj67_lNzLT78_syV1IBymRvgUKh6mJwyh8zkX3F8iEX7j1YfGu0NVKfQxG50BEafrBwvSLUHQxPbxAzRdFdMitsiqDwa6E/s200/body-verde.png" /></a>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#0080FF'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgDxdpA3kzI3-mq_Q2Fd7rbkjVMXTwUKO20PEy2x5e5KqAtiVOnXbRoEHt-PRPPG9pe4d9Pkf1PAnqWSinEKNVnhDriRCR60SllnVenDT5AgfDqXtsOaVww53ryd5lAu_gwotP47kOS746z/s200/body-azul.png" /></a>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#CC00FF'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjrYg4SnUn833Nskm9kONWOe5hsVIfeKMcd2Yf1-0kFr5zs1fbDhwn3lLn4Goqb5cL8lH7a4I0EeBygQ1VtFpe9178SsJvFcxl-ZdydS1ElLf-CYfjHnmtOeC7Ay-_7ILDP72Q5ReBMZ2vT/s200/body-morado.png" /></a>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#F781BE'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjhhkG3UcAd-cBBGPoT8PASgLD6VPhaAi5KR4ESTXIbyq6Gm4vxs6HXbOJg-btJ1BMrOyn4-3M_dSgv8HC7v3Q7utXqFtkOhjVpni-w3wJ_pn4Uh1CnPKqY40VWLCbES4hHGR6NDdxMKIZU/s200/body-rosa.png" /></a>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#000000'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjhEC8-Vx-vNr65xBBXRvYltYZYQgJrNvLL3xLEgweawKqDsE4HzuTn1aQHmetd33wkEcZ4oymwq3RCm_ZY987gUbvsG0zeMGt1JQo4TCtv-YniXQ7jXxjTSNxkD9B9qcsHum-w_tfHygcD/s200/body-negro.png" /></a>
<a href="#" onclick="javascript:document.body.style.backgroundColor='#FFFFFF'; document.body.style.backgroundImage='none';"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghKfUjGoGmq-rwQc1OWTOV6TD44eycxAndZiOL3H4J5vW8E1VSplqturEwQYKqH-KNFNZAEAVI_oq78QopGzdfRX6mXyCPm8y2cp60-uPWa-we_mEf3dxtj-ZwjpC0ac8GWe0t5ujrqW4y/s200/body-blanco.png" /></a>
</center>

Para o resultado 2, cole:

 <div class='widget HTML' id='HTML7'>
<h2 class='title'></h2>
<div class='widget-content'>
<style>
#backgrounds a img{
width:40px;
height:40px;
display:inline;
margin-right:10px;
margin-top:15px;
border:2px solid #FFFFFF;
}
#backgrounds a:hover img{
border:2px solid #FFEC8B;
}
</style>
<div id="backgrounds">
<script src='http://code.jquery.com/jquery-1.6.2.min.js' type='text/javascript'></script>
<script>
$(document).ready(function(){
 $("#background01").click(function(){
$("body").css({"background":"url(URLDOFUNDO1)"});
 });
});
$(document).ready(function(){
 $("#background02").click(function(){
$("body").css({"background":"url(URLDOFUNDO2)"});
 });
});
$(document).ready(function(){
 $("#background03").click(function(){
$("body").css({"background":"url(URLDOFUNDO3)"});
 });
});
</script>
<center><a href="javascript:void(O)" id='background01' title="1"><img src="URLDOFUNDO1" /></a>
<a href="javascript:void(O)" id='background02' title="2"><img src="URLDOFUNDO2" /></a>
<a href="javascript:void(O)" id='background03' title="3"><img src="URLDOFUNDO3" /></a></center></div>
</div></div>

Substitua os trechos em vermelho pelo background 1, os trechos em rosa pelo background 2 e os trechos em azul pelo background 3.

3. Salve.