Dynamically add HTML to ASP.NET page

Could someone please advise what the "correct" method is for adding HTML content to an ASP.NET page dynamically?

I am aware of the following declarative method.

//Declaration
<%= MyMethodCall() %>


//And in the code behind.
protected String MyMethodCall()
{
    return "Test Value";
}

Is there a better or best practice way?

EDIT: I am building a Galleriffic photo gallery dynamically depending on the images located in a specific folder.