Dynamically creating elements with an object constructor
I am going to answer the questions in reverse order and share some design thoughts at the end.




  Can the function call be integrated either into the constructor function (and called when a new object is created) or called from the new object parameter? (It somehow feels wrong to place on a separate line after every new object creator.


Yes; it can.

The constructor function is just that; a function. In fact, it doesn't even need to be: if you move the logic from Photo.create into the constructor (which seems like a valid decision), then there is no point in this being it's own class.

To move the Photo.create logic into the constructor, just move it up a scope:

function Photo(name, src) {
    this.name = name;
    this.src = src;
    var container = document.getElementById('thumbs_container'); 
    var img = document.createElement('img');
    img.src = this.src;
    img.alt = this.name;
    img.className = 'thumb';
    img.style.width = '200px';
    container.appendChild(img);
}


However, as I stated above, this doesn't really need to be a class anymore; it's just one function.




  Is the code valid and are there any main reasons not to use this technique?


No, the way you are doing this is just fine. However, as stated above, it does not need to be object-oriented for the implementation you have provided.

If your code is going to specific to what you have shown where you paint the picture as soon as you create, then this ought to be a function.



However, there is still a way that this can be improved. If you are familiar with regexes, then you know that there is such thing as compiling a regex before you actually use it. The reason for this is because compiling regexes can be a time and resource consuming process, so it's best to get it out of the way so it does not slow the the code in the middle of it.

You can do something similar here: if, in the constructor function, you..


Create the element
Define the container
Define the class
And define the width


then, you can just store this information for when you want to put the image on the DOM.

To use this, you would create a new Photo instance which would do the above in advance. Then, later in your code when you are ready to paint, you can specify (or re-specify) the source and name.

The constructor would look like this:

function Photo(name, src) {
    this.name = name || "";
    this.src = src || "";

    var container = document.getElementById('thumbs_container'); 
    var img = document.createElement('img');
    img.src = this.src;
    img.alt = this.name;
    img.className = 'thumb';
    img.style.width = '200px';

    // no painting yet

    this.create = function() {
        container.appendChild(img);
    }
}


Note how this code looks almost identical to the first code snippet in this answer. The only difference here is that you are preparing the system resources ahead of time; just like regexes. This will provide a speed boost in your code if used correctly.

var photo = new Photo(); // prepare resources for image

/* bunch of code all up until when it is needed to draw */

photo.create(); // put image/resources in DOM