Just place this code into your Ning appearance (old and new editor supported). it will make your blog posts images look like this:
/*--Blog images effect--*/.xg_widget_profiles_blog_show .postbody img{ background: none repeat scroll 0 0 #FFFFFF; border: 1px solid #CECECE; padding: 4px !important; box-shadow: 0 0 6px rgba(0, 0, 0, 0.2);}.xg_widget_profiles_blog_list .postbody img{ …