It is not possible, the image and text are separate and placed according to the size of the browser. The height of the image is unknown, but will always be in the center.
However, the text can be placed on the bottom of the screen if the styles in css/wl-sytle.css
are changed.
#loading{
position:absolute;
left:0;
right:0;
bottom:25px;
text-align:center;
color:white;
}