The *src keys inside plotly graphs are not used for images or links (unlike the html.A src attribute).
The currently is not a way to include custom symbols, you are limited to the symbols that are provided. Here is where all of these attributes are documented: https://plot.ly/python/reference/#scattermapbox-marker-symbol
And here is a full list of the available scattermapbox symbols:
airfield
airport
alcohol-shop
amusement-park
aquarium
art-gallery
attraction
bakery
bank
bar
beer
bicycle
bicycle-share
bus
cafe
campsite
car
castle
cemetery
cinema
circle
circle-stroked
clothing-store
college
dentist
doctor
dog-park
drinking-water
embassy
entrance
fast-food
ferry
fire-station
fuel
garden
golf
grocery
harbor
heliport
hospital
ice-cream
information
laundry
library
lodging
marker
monument
mountain
museum
music
park
pharmacy
picnic-site
place-of-worship
playground
police
post
prison
rail
rail-light
rail-metro
religious-christian
religious-jewish
religious-muslim
restaurant
rocket
school
shop
stadium
star
suitcase
swimming
theatre
toilet
town-hall
triangle
triangle-stroked
veterinary
volcano
zoo