The line webshot("tmp.html","fig_1.png", delay =5, vwidth = 480, vheight=480) # changed to png. will save an imaged named fig_1.png to your working directory.
You can check where your working directory with;
getwd()
#> [1] "/Users/notarobot/rstudio_git/community-sandbox"
Created on 2018-08-10 by the reprex package (v0.2.0.9000).
Here's a userful guide on working directories and workspaces with r and rstudio, Working Directories and Workspaces, by RStudio Support