Could you share the code you're trying to use to add the markers? It helps us help you to see what you're working with.
For example, below I've changed the example from the link you gave to use a little picture of an angler fish (note that I've changed the dimensions of the image accordingly, but haven't changed anything else, so you still have the shadow image from the greenLeafIcon example)
library(leaflet)
data(quakes)
leaflet(data = quakes[1:20,]) %>% addTiles() %>%
addMarkers(~long, ~lat, popup = ~as.character(mag), label = ~as.character(mag))

anglerIcon <- makeIcon(
iconUrl = "https://i.imgur.com/otS5q0K.png",
iconWidth = 64, iconHeight = 64,
iconAnchorX = 22, iconAnchorY = 94,
shadowUrl = "http://leafletjs.com/examples/custom-icons/leaf-shadow.png",
shadowWidth = 50, shadowHeight = 64,
shadowAnchorX = 4, shadowAnchorY = 62
)
leaflet(data = quakes[1:4,]) %>% addTiles() %>%
addMarkers(~long, ~lat, icon = anglerIcon)

Created on 2019-06-18 by the reprex package (v0.3.0)