[website_blog] mobile: footer image Issue

fix a small issue in website_blog mobile display by improving css.
More Details
Task: https://openerp.my.openerp.com/web#id=8988&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.m0zuVDWbrWoKc7Iv