如何在Markdown中为ioslides_presentation添加图像作为页脚

Question

您好我想将图片作为页脚添加到ioslides_presentation shiny widgets。我试图重现这个example，但它没有用。我想将此.png文件添加为页脚：

---
title: ""
author: ""
date: ""
output: 
  ioslides_presentation
header-includes:
    - \usepackage{wallpaper}
runtime: shiny
---
\LRCornerWallPaper{0.99}{footer.png}