I have the following code
myicon <- makeIcon( iconUrl = "myicon.ico" )
html_legend <- "<img src='myicon.ico'>hey<br/>"
map_leaflet <- leaflet() %>%
addMarkers(50, 50, icon = myicon, popup = paste("geo")) %>%
addControl(html = html_legend, position = "bottomright")
The question mark error icon means that it cannot find your image.
This is normal since during creation Leaflet is not running on a webserver. When you place your generated HTML-file on a webserver and the image icon in the appropriate place it will work.
I did this myself. Same error icon and worked like a charm when I placed it on the webserver. A page that uses the same trick.