I'm starting to use the Tufte package, and I'm trying to plot an image read from a png file in full.width, I tried using the 
approach, which plots it in the main margin (not full width), also tried reading with imager::load.image(image.png)
and plotting within a {r fig.fullwidth = TRUE}
call, but it just centers it and leaves for a lot of tweaking to get the figure into full width, besides leaving a lot of space in a page.
Using include_graphics does the trick:
{r fig-fullwidth, fig.width = 10, fig.height = 2, fig.fullwidth = TRUE, fig.cap = "A full width figure.", warning=FALSE, message=FALSE, cache=TRUE, out.width='100%'}
knitr::include_graphics("agua_sla_fuentes_corte.png")
and it also works for margin figures