Use external markers by creating icons using icons() function.
Ex:
smile = makeIcon(iconUrl = "smilie.png", iconHeight, iconWidth, ...)
or use icons () function
smile = icons (iconUrl = "smilie.png", iconHeight, iconWidth, ...)
Use the created icon as a marker by providing the icon attribute as the above created variable.
leaflet(data = df) %>% addTiles() %>% addMarkers(~long, ~lat, icon = smile)