Create a Legend on a Folium map

You can add a legend quite easily;

#specify the min and max values of your data
colormap = branca.colormap.linear.YlOrRd_09.scale(0, 8500)
colormap = colormap.to_step(index=[0, 1000, 3000, 5000, 8500])
colormap.caption = 'Incidents of Crime in Victoria (year ending June 2018)'
colormap.add_to(world_map)

You can see my complete example here;

Folium Map with legend example

Folium now has a way to add an image easily with version 0.15.

from folium.plugins import FloatImage
image_file = 'image.PNG'

FloatImage(image_file, bottom=0, left=86).add_to(mymap)