add and resize a local image to a .Rmd file in RStudio that will produce a pdf

From @tmpname12345

You can use raw latex to include a figure in pdf_output: \includegraphics[width=250pt]{path/file.png}

You can also specify the size of the image like so:

![](filepath\file.jpg){ width=50% }

The width and height attributes on images are treated specially. When used without a unit, the unit is assumed to be pixels. However, any of the following unit identifiers can be used: px, cm, mm, in, inch and %. There must not be any spaces between the number and the unit.

Source: Pandoc's RMarkdown Documentation - Images

In case anyone arrives here from google looking to insert an image into an RMarkdown html_document:

Insert directly

This method is arguably the easiest to change size

<img src="mypic.png" alt="drawing" width="200" height="50"/>

Another way

Note you can mix measurements like so: height="200" width=60%

![some caption text here](mypic.png){height="200" width=60% }

Insert via RMarkdown chunk

knitr::include_graphics("mypic.png")

Insert directly from URL

```{r echo=FALSE, out.width = '60%'}
image_url <- "http://www.example.com/mypic.png"
```
<center><img src="`r image_url`"></center>