title: bookdown issue - knitr::include_graphics() - "Unable to load picture or PDF file" using image URL
body: <p><strong>ETA:  I don't know if this makes a difference, but a couple days ago, I upgraded/updated my Linux packages.  I have had issues in the past where that causes some issues in R/RStudio.</strong></p>
  
  <p>Strange happenings with bookdown.  Just a couple days ago, bookdown was building my book beautifully.  Now, when I try to build my book, it seems that <code>knitr::include_graphics()</code> is creating an issue, but only if it points to an image URL.</p>
  
  <p>I created a <em>new</em> bookdown project to see if the problem would persist; it did.  bookdown built Yihui's minimal example perfectly.  Then, I used <code>knitr::include_graphics()</code> to add an image file in the book's directory.  Again, in that case, bookdown built the book, and everything worked just right.  But, when I changed the image location to a URL, the error message reads:</p>
  
  <blockquote>
    <p>! LaTeX Error: File
    `<a href="https://upload.wikimedia.org/wikipedia/commons/2/2e/Charle" rel="nofollow noreferrer">https://upload.wikimedia.org/wikipedia/commons/2/2e/Charle</a>
    s_Darwin_seated_crop' not found.</p>
    
    <p>Error: Failed to compile testbook.tex. See testbook.log for more info.
    Execution halted</p>
    
    <p>Exited with status 1.</p>
  </blockquote>
  
  <p>I am at a loss.  The only thing I'm changing is whether the <code>include_graphics()</code> command is using a local file versus one at a URL.  Any suggestions are appreciated.  Here is my session info:</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.4.4 (2018-03-15)
  Platform: x86_64-pc-linux-gnu (64-bit)
  Running under: Linux Mint 18
  
  Matrix products: default
  BLAS: /usr/lib/libblas/libblas.so.3.6.0
  LAPACK: /usr/lib/lapack/liblapack.so.3.6.0
  
  locale:
   [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
   [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
   [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
   [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
   [9] LC_ADDRESS=C               LC_TELEPHONE=C            
  [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
  
  attached base packages:
  [1] stats     graphics  utils     datasets  grDevices
  [6] methods   base     
  
  other attached packages:
   [1] ggthemr_1.1.0            mapdata_2.3.0           
   [3] maps_3.3.0               BRugs_0.9-0             
   [5] bookdown_0.8.2           blogdown_0.9            
   [7] rasterImage_0.3.0        plotrix_3.7-4           
   [9] rasterVis_0.45           latticeExtra_0.6-28     
  [11] lattice_0.20-38          gridExtra_2.3           
  [13] rgeos_0.4-2              dismo_1.1-4             
  [15] RColorBrewer_1.1-2       ggthemes_4.0.1          
  [17] R2OpenBUGS_3.2-3.2       popbio_2.4.4            
  [19] demogR_0.6.0             png_0.1-7               
  [21] jpeg_0.1-8               rnaturalearthhires_0.1.0
  [23] rnaturalearthdata_0.1.0  rnaturalearth_0.1.0     
  [25] raster_2.8-4             rgdal_1.3-6             
  [27] scales_1.0.0             ggmap_2.7.904           
  [29] ggvis_0.4.4              rworldmap_1.3-7         
  [31] maptools_0.9-4           sp_1.3-1                
  [33] googlesheets_0.3.0       forcats_0.3.0           
  [35] stringr_1.3.1            dplyr_0.7.8             
  [37] purrr_0.2.5              readr_1.3.0             
  [39] tidyr_0.8.2              tibble_1.4.2            
  [41] ggplot2_3.1.0            tidyverse_1.2.1         
  
  loaded via a namespace (and not attached):
   [1] nlme_3.1-137      bitops_1.0-6      sf_0.7-1         
   [4] lubridate_1.7.4   httr_1.4.0        tools_3.4.4      
   [7] backports_1.1.2   R6_2.3.0          spData_0.2.9.6   
  [10] DBI_1.0.0         lazyeval_0.2.1    colorspace_1.3-2 
  [13] withr_2.1.2       tidyselect_0.2.5  compiler_3.4.4   
  [16] cli_1.0.1         rvest_0.3.2       xml2_1.2.0       
  [19] hexbin_1.27.2     classInt_0.2-3    digest_0.6.18    
  [22] foreign_0.8-70    rmarkdown_1.11    pkgconfig_2.0.2  
  [25] htmltools_0.3.6   rlang_0.3.0.1     readxl_1.1.0     
  [28] rstudioapi_0.8    shiny_1.2.0       bindr_0.1.1      
  [31] generics_0.0.2    zoo_1.8-4         jsonlite_1.6     
  [34] magrittr_1.5      dotCall64_1.0-0   Rcpp_1.0.0       
  [37] munsell_0.5.0     yaml_2.2.0        stringi_1.2.4    
  [40] plyr_1.8.4        grid_3.4.4        parallel_3.4.4   
  [43] promises_1.0.1    crayon_1.3.4      haven_2.0.0      
  [46] hms_0.4.2         knitr_1.21        pillar_1.3.0     
  [49] boot_1.3-20       rjson_0.2.20      codetools_0.2-15 
  [52] glue_1.3.0        evaluate_0.12     modelr_0.1.2     
  [55] spam_2.2-0        httpuv_1.4.5      RgoogleMaps_1.4.3
  [58] cellranger_1.1.0  gtable_0.2.0      assertthat_0.2.0 
  [61] xfun_0.4          mime_0.6          xtable_1.8-3     
  [64] broom_0.5.1       e1071_1.7-0       coda_0.19-2      
  [67] later_0.7.5       class_7.3-14      viridisLite_0.3.0
  [70] fields_9.6        units_0.6-2       bindrcpp_0.2.2 
  </code></pre>