How to define unchecked value to checkbox?

I'm trying to set a default value for 'unchecked', but is not working. The inputs always saves with 'yes' value.

<input type="hidden" name="imagem" value="no" />
<input id="imagem" type="checkbox" value="yes" >Imagem<br />

<input type="hidden" name="som" value="no" />
<input id="som" type="checkbox" value="yes" >Som<br />

<input type="hidden" name="animacao" value="no" />
<input id="animacao" type="checkbox" value="yes" >Animação<br />