The animated loading gif is located at quform/images/default-loading.gif. Replace that image with your own. If the image doesn’t change for you do a force reload in the browser by pressing Ctrl+F5.
You might also want to adjust the CSS for the new image, you can do so in the quform/css/base.css file, at these selectors:
.quform-loading-wrap {
.quform-loading-wrap .quform-loading {