Edit in JSFiddle

// the class
Request.flickr = new Class({
    Extends: Request.JSONP,
    options: {
        callbackKey: "jsoncallback",
        url: "http://www.flickr.com/services/rest/?",
        log: true
    },
    initialize: function(params, options) {
        this.parent(options);
        this.options.url = this.options.url + Object.toQueryString(params);
    },
    success: function(data, script) {
        this.parent(data, script);
    },
    imageURL: function(obj) {
        return "http://farm{farm}.static.flickr.com/{server}/{id}_{secret}.jpg".substitute(obj);
    }
});

// how to use
new Request.flickr({
    format: 'json',
    api_key: "e7df6c74d2545f55414423463bf99723", // your api here
    per_page: 4,
    tags: "mountains",
    method: "flickr.photos.search"
}, {
    onSuccess: function(data) {
        target = $("action");
        var self = this;
        data.photos.photo.each(function(el) {
            new Asset.image(self.imageURL(el), {
                onload: function() {
                    this.inject(target);
                }
           });
        });
    }
}).send();

<div id="action">ready</div>