Custom Marker Tooltips

Setting custom content in a marker tooltip
<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>Custom Marker Tooltips</title>
  <meta name='viewport' content='width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no' />
  <script src='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.js'></script>
  <link href='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.css' rel='stylesheet' />
  <!--[if lte IE 8]>
    <link href='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.ie.css' rel='stylesheet'>
  <![endif]-->
  <style>
    body { margin:0; padding:0; }
    #map { position:absolute; top:0; bottom:0; width:100%; }
  </style>
</head>
<body>
<div id='map'></div>
<style>
.popup {
    text-align: center;
}
.popup img {
    width: 100%;
}
</style>
<script type='text/javascript'>
var map = L.mapbox.map('map', 'examples.map-9ijuk24y');

var geoJson = [{
    type: 'Feature',
    "geometry": { "type": "Point", "coordinates": [-77.03, 38.90]},
    "properties": {
        "image": "https://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Cherry_Blossoms_and_Washington_Monument.jpg/320px-Cherry_Blossoms_and_Washington_Monument.jpg",
        "url": "https://en.wikipedia.org/wiki/Washington,_D.C.",
        "marker-symbol": "star",
        "city": "Washington, D.C."
    }
}, {
    type: 'Feature',
    "geometry": { "type": "Point", "coordinates": [-87.63, 41.88]},
    "properties": {
        "image": "https://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Chicago_sunrise_1.jpg/640px-Chicago_sunrise_1.jpg",
        "url": "https://en.wikipedia.org/wiki/Chicago",
        "city": "Chicago"
    }
}, {
    type: 'Feature',
    "geometry": { "type": "Point", "coordinates": [-74.00, 40.71]},
    "properties": {
        "image": "https://upload.wikimedia.org/wikipedia/commons/thumb/3/39/NYC_Top_of_the_Rock_Pano.jpg/640px-NYC_Top_of_the_Rock_Pano.jpg",
        "url": "https://en.wikipedia.org/wiki/New_York_City",
        "city": "New York City"
    }
}];

// Add custom popups to each using our custom feature properties
map.markerLayer.on('layeradd', function(e) {
    var marker = e.layer,
        feature = marker.feature;

    // Create custom popup content
    var popupContent =  '<a target="_blank" class="popup" href="' + feature.properties.url + '">' +
                            '<img src="' + feature.properties.image + '">' +
                        '   <h2>' + feature.properties.city + '</h2>' +
                        '</a>';

    // http://leafletjs.com/reference.html#popup
    marker.bindPopup(popupContent,{
        closeButton: false,
        minWidth: 320
    });
});

// Add features to the map
map.markerLayer.setGeoJSON(geoJson);

map.setView([45.908, -78.525], 4);
</script>
The code and documentation to mapbox.js is hosted on GitHub where you can contribute changes and improvements.