There is a example how to add country flags to checkBoxGroupInput here
https://gist.github.com/bborgesr/f2c865556af3b92e6991e1a34ced2a4a
I am trying to adjust the code slightly to achieve the same result using pickerinput from shinywidgets. However, in my results I do not see any image. 
  library(shiny)
  library(shinyWidgets)
  countries <- c("Australia", "United Kingdom", "United States")
  flags <- c(
    "https://cdn.rawgit.com/lipis/flag-icon-css/master/flags/4x3/au.svg",
    "https://cdn.rawgit.com/lipis/flag-icon-css/master/flags/4x3/gb.svg",
    "https://cdn.rawgit.com/lipis/flag-icon-css/master/flags/4x3/us.svg"
  )
  ui <- fluidPage(
    pickerInput("countries", "countries", multiple = T,
                choices = countries,
                choicesOpt = list(content =  
                                    mapply(countries, flags, FUN = function(country, flagUrl) {
                                      tagList(
                                        tags$img(src=flagUrl, width=20, height=15),
                                        country
                                      )
                                    }, SIMPLIFY = FALSE, USE.NAMES = FALSE)
                                    ))
    ,
  textOutput("txt")
  )
    server <- function(input, output, session) {
    output$txt <- renderText({
      paste("You chose", paste(input$countries, collapse = ", "))
    })
  }
  shinyApp(ui, server)
Hi you don't want to add options as tagList but rather as HTML strings like this
library(shiny)
library(shinyWidgets)
countries <- c("Australia", "United Kingdom", "United States")
flags <- c(
  "https://cdn.rawgit.com/lipis/flag-icon-css/master/flags/4x3/au.svg",
  "https://cdn.rawgit.com/lipis/flag-icon-css/master/flags/4x3/gb.svg",
  "https://cdn.rawgit.com/lipis/flag-icon-css/master/flags/4x3/us.svg"
)
ui <- fluidPage(
  pickerInput("countries", "countries", multiple = T,
              choices = countries,
              choicesOpt = list(content =  
                                  mapply(countries, flags, FUN = function(country, flagUrl) {
                                    HTML(paste(
                                      tags$img(src=flagUrl, width=20, height=15),
                                      country
                                    ))
                                  }, SIMPLIFY = FALSE, USE.NAMES = FALSE)
              ))
  ,
  textOutput("txt")
)
server <- function(input, output, session) {
  output$txt <- renderText({
    paste("You chose", paste(input$countries, collapse = ", "))
  })
}
shinyApp(ui, server)
Hope this helps!
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With