Apart from syntax, I fail to see how this code:

    getPhotos tags =
        let photoList  = send (lift requestTag tags) in
        let photoSizes = send (lift requestOneFrom photoList) in
            lift sizesToPhoto photoSizes

...is any clearer than the equivalent callback code:

  function getPhoto(tag, handlerCallback) {
        asyncGet(requestTag(tag), function(photoList) {
            asyncGet(requestOneFrom(photoList), function(photoSizes) {
                handlerCallback(sizesToPhoto(photoSizes));
            });
        });

In particular, you still have to nest the elm signals in 'let ... in' blocks.