.pdf image files were not rendering in html, converted to .png and modified source to reflect change. Left .pdfs in img/Fixes issue 160

Author: breckbaldwin

src/stan-users-guide/img/change-point-posterior.png

@@ -236,14 +236,14 @@ default MCMC implementation is shown in the posterior plot.
 Log probability of change point being in year, calculated analytically.
 
 ```{r include=TRUE, fig.align="center", fig.cap=c("Analytical change-point posterior"), echo=FALSE}
-knitr::include_graphics("./img/change-point-posterior.pdf", auto_pdf = TRUE)
+knitr::include_graphics("./img/change-point-posterior.png", auto_pdf = TRUE)
 ```
 
 The frequency of change points generated by sampling the discrete change
 points.
 
 ```{r include=TRUE, fig.align="center", fig.cap=c("Sampled change-point posterior"), echo=FALSE}
-knitr::include_graphics("./img/s-discrete-posterior.pdf", auto_pdf = TRUE)
+knitr::include_graphics("./img/s-discrete-posterior.png", auto_pdf = TRUE)
 ```
 
 In order their range of estimates be visible, the first plot is on the log