Para centralizar basta voce editar o html e fazer da seguinte maneira adicionar <center>? Texto/imagem ou código </center>
Feito isso salve as configurações .
Exemlo
<center><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6QwMZPNuXNF0Y1Zppv03R8l8rzyUwBDLra2t3xhg3vIkBQ9GfD_6Fs2jtpxg-wtQ4f6k-6p7tdojZEjL4Ow4-GQqVEMN_7KPoFHADiWjdUgIcDHCL_KE9x9hBrcknD_m6nPdnlp4oEx0g/s1600/Cubo-magico.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6QwMZPNuXNF0Y1Zppv03R8l8rzyUwBDLra2t3xhg3vIkBQ9GfD_6Fs2jtpxg-wtQ4f6k-6p7tdojZEjL4Ow4-GQqVEMN_7KPoFHADiWjdUgIcDHCL_KE9x9hBrcknD_m6nPdnlp4oEx0g/s320/Cubo-magico.jpg" width="320" /></a></center>
