Change background in R Markdown beamer presentation

I'm writing on a beamer presentation in rmarkdown and I have two types of frames which should differ by their background. So I wrote two functions like that in latex:

\newcommand{\settitlestyle}{
\setbeamertemplate{background canvas}{%
  \includegraphics[width = \paperwidth, height = \paperheight] 
{backgroundtitle.jpg}}
} 

\setmainstyle is exactly the same command but another jpg.

In the YAML I have already input a tex file that defines the functions and calls \settitlestyle. Works. But after the first slide I want to switch to the mainstyle. When I call \setmainstyle in the markdownfile nothing happens.