Currently you can't add a custom image on the front page. However, if you are using the registration module, you can set a splash image in the module settings.

Hey there,

I am really sorry for the delay of this reply as it is a weekend for us here.

Currently that was not possible to add image into login/registration page, but you can modify on this file -> JoomlaFolder\components\com_easysocial\themes\wireframe\login\default.php

Or you can use this Easysocial quick registration module.