The old compare to the new, A portrait of Vangogh (left) and William-Aldolphe Bougouere (right)
these are both self portraits however.
Context
Pretty much everywhere we go nowadays you can find portraiture, many businesses and companies will use it to promote products they sell, using famous people to make them more popular. We find them in leaflets, on billboards, magazines and in newspapers mainly. Celebrities tend to have their own websites for the public to see so these are the likeliest places to find them. An advantage of portrait is that it uses less space so this is why many magazines and newspapers will use them over landscape.
Techniques
One technique is the texture, in fact this is a big part of it. it is used to make many images seem to jump out at you from the page. Matching the background texture to the foreground is important if you are to use this technique, otherwise it will look strange.