There are different ways to include a picture in Rmarkdown. The basic way is to use ![caption](path/to/picture.extension). If the figure is not in an bookdown theorem environment, you may also use knitr.

```{r figure, echo=FALSE}
knitr::include_graphics(path/to/picture.extension)
```

The way using knitr to include graphics provide more options to control the dimension and position of a figure. For details, you may read Yihui’s knitr manual on plots.

Because I normally have to include pictures in theorem environment in bookdown. The knitr function is not an option for me to include a picture. However, that’s not the end. We only need some external tools to get what we want.

Centering and Scaling for HTML Output

To center and change the dimension globally for pictures for the html output, we may add the following css code chunk at the beginning of your Rmarkdown file.

```{css, include=FALSE}
img {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width:60%;
    max-width:360px;
}
```

Centering and Scaling for PDF Output

For PDF (using TeX) output, to change dimension, first you need create a tex file, say preamble.tex, and add the following into it.

\usepackage{graphicx}
%The following codes scale images in mainmatter if necessary
%It will be overwrite by local setting in \includegraphics[width, height, ...]{}
\makeatletter
\def\maxwidth{\ifdim\Gin@nat@width>\textwidth \textwidth\else\Gin@nat@width\fi}
\def\maxheight{\ifdim\Gin@nat@height>0.5\textheight 0.5\textheight\else\Gin@nat@height\fi}
\makeatother
\setkeys{Gin}{width=\maxwidth,height=\maxheight,keepaspectratio}
%The following codes add max dimension options to includegraphics
\usepackage[export]{adjustbox} %Needed for max width
\usepackage{etoolbox} %Needed for \pathcmd
\expandafter\patchcmd\csname Gin@ii\endcsname
{\setkeys{Gin}{#1}}
{\setkeys{Gin}{max width=\textwidth, max height=.5\textwidth,keepaspectratio,#1}}
{}
{}

Then add the following to Rmarkdown yaml under the output for pdf.

includes:
    in_header: latex/preamble.tex

To center pictures for PDF, we need to add \centering to each picture environment. One way is to use bookdown.post.latex. An example of usage of bookdown.post.latex can be found in my previous post. Another way, which works better is to use a pandoc lua filter. First, you need create a file, add the following codes and save as a .lua file, say latex.lua.

function Image (el)
    if el.classes:includes("center") then
    return {
      pandoc.RawInline('latex', '\\hfill\\break{\\centering'),
      el,
      pandoc.RawInline('latex', '\\par}')
    }
  end
end

Then add the following option to the Rmarkdown YAML under the output for pdf.

pandoc_args: "--lua-filter=latex/latex.lua"

Edited on 2021/06/09