BuddyPress.org

This line is missing from your screen.css

.widget_bp_core_widget_whos_online .item-avatar {

float: left;

margin-right: 10px;

}

The avatars are not floated left, which should be as per the default theme, if you want that layout.

There may be other css omissions too, so I will advise take a look at the css file.