You must use "url" hash key instead of "picture"
@hash = Gmaps4rails.build_markers(@additional_infos) do |additional_info, marker|
marker.lat additional_info.latitude
marker.lng additional_info.longitude
marker.picture({
"url" => view_context.image_path('/qr-code-icon.png'),
"width" => 32,
"height" => 37
})
end