Hi @Amarakon,
There seems to be many ways to set backgrounds using LaTeX: graphics - How to use background image in LaTeX? - TeX - LaTeX Stack Exchange
So you just need to add some code to the preamble and then indicate which page should have a background. Note that image.jpeg is an image in the same directory as the Rmd file.
---
title: "Untitled"
date: "`r Sys.Date()`"
output: pdf_document
header-includes:
- \usepackage[pages=some]{background}
- \backgroundsetup{scale=1,opacity=1,angle=0,contents={\includegraphics[width=\paperwidth,height=\paperheight]{image.jpeg}}}
---
# Important Section
Here is some important text.
\pagebreak
# Last Page
\BgThispage