Just place it as a background image on .wpa-middle-slogan and place it like 30 pixels from the bottom. Report back if you have any issues.
CSS3 multiple background images isn’t supported in IE8 so if you did want to add in support for that, create another container element inside of .wpa-middle-slogan and have that element hold the image.
The arrow image would be perfect for SVG, rather than a giant image with loads of transparent pixels and probably relying on scaling algorithms to avoid an ugly pixilated look. Again, IE8 doesn’t support SVG though.
I suppose one of these tricks could be considered: