I was able to reproduce similar issue if i clicked the 'Run App' button, but if I use runApp() directly in the concole, the picture can be displayed as expected, not sure why though...
ps. my current working directory is the same as the file path of app.R
library(shiny)
library(shinydashboard)
title <- tags$a(href='https://google.com',
tags$img(src='b.jpg', width='100px',height='100px')) # an image downloaded from web (b.jpg under www folder)
header <- dashboardHeader( title = title, uiOutput("logoutbtn"))
ui <- dashboardPage(header, dashboardSidebar(), dashboardBody())
server <- function(input, output, session) {
}
shinyApp(ui, server)