Initials for Avatar

I don't know if there is such an API, but this can be done in various ways. You can do it both on the frontend or the backend. For instance, in PHP, you could use the GD library to check for the existence of the user image, and if it doesn't exist, generate one with GD so that it contains the initial, and serve that newly generated image instead of the profile photo.

On the frontend side, let's say you're retrieving user data as a JSON. Once you parse the JSON, you'll have an object which could look like this:

var user = {
    username: "jwitz",
    name: "Justin Witz",
    photo: "/path/to/photo" // or an empty string if there is no photo
}

In this scenario, you can check user.photo and if it's an empty string, you'd populate the div (or whatever element) reserved for the profile photo with a specially styled element that displays the initial and set its innerHTML to user.name[0];

Or you could use a canvas element and "draw" the letter on the canvas, followed by the image over it, if it exists.

If the user images are served from a predefined uniform url (like user/username/photo) you can even use an XMLHttpRequest to check if the image exists or the path returns a 404 in which case you'll apply the first letter of the name or surname.

If you use jQuery, something like this could work:

$.get('user/' + user.username + '/photo')
.done(function() { 
    // add the image to the DOM
}).fail(function() { 
    // add the specially styled element and set its text to the first letter of the name
});

I hope this helps, or at least gives you a general idea about how it can be done.