title: Icons not loading (empty image) in R Leaflet with Shiny
body: <p>[R-3.4.3 64-bit, RStudio, shinydashboard_0.6.1, shiny_1.0.5, leaflet.extras_0.2, Chrome] </p>
  
  <p>I'm making icons to use in R/Leaflet with Shiny and all im getting is the below, but i've no idea why:</p>
  
  <p><a href="https://i.stack.imgur.com/9RCBg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9RCBg.png" alt="enter image description here"></a></p>
  
  <p>This is using the <a href="http://rstudio.github.io/leaflet/markers.html" rel="nofollow noreferrer">toy example from here</a>:</p>
  
  <pre><code>oceanIcons &lt;- iconList(
    ship = makeIcon("ferry-18.png", "ferry-18@2x.png", 18, 18),
    pirate = makeIcon("danger-24.png", "danger-24@2x.png", 24, 24)
  )
  
  # Some fake data
  df &lt;- sp::SpatialPointsDataFrame(
    cbind(
      (runif(20) - .5) * 10 - 90.620130,  # lng
      (runif(20) - .5) * 3.8 + 25.638077  # lat
    ),
    data.frame(type = factor(
      ifelse(runif(20) &gt; 0.75, "pirate", "ship"),
      c("ship", "pirate")
    ))
  )
  
  leaflet(df) %&gt;% addTiles() %&gt;%
    # Select from oceanIcons based on df$type
    addMarkers(icon = ~oceanIcons[type])
  </code></pre>
  
  <p>And the following, with different but similar toy data, when using <code>runApp(shinyApp(ui, server), launch.browser = TRUE)</code>;</p>
  
  <p><a href="https://i.stack.imgur.com/GdZkF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GdZkF.png" alt="enter image description here"></a></p>