Don't show code when including graphics

intro.Rmd

@@ -109,7 +109,7 @@ To run the code in this book, you will need to install both R and the RStudio ID
 
 RStudio is an integated development environment, or IDE, for R programming. There are three key regions:
 
-```{r}
+```{r, echo = FALSE}
 knitr::include_graphics("screenshots/rstudio-layout.png")
 ```
 
@@ -129,7 +129,7 @@ If you want to see a list of all keyboard shortcuts, use the meta keyboard short
 
 We strongly recommend making two changes to the default RStudio options:
 
-```{r}
+```{r, echo = FALSE}
 knitr::include_graphics("screenshots/rstudio-workspace.png")
 ```