The model is crucial in the fashion images, the focus is usually on them so its important that they are the right person for the job (particularly in simple studio shots where there isnt alot going on).
There are always trends when it comes to models but today one of the most important things is that they are versatile, they have to be able to carry off different looks at opposite ends of the scale. They almost have to be like actors/actresses to fit into the different characters/genres.
The model must communicate the right thing within the image, for instance many fashion images work on the basis of aspirations whether thats lifestyle or body image the model must fit.
These images are of model Gemma Ward, she has been used for a range of runway, editorial and advertising and is incredibly versatile.
These images are of Rosie Huntington-whiteley, she is most famous for being a 'Victorias secret' model but she has also been used for fashion and other campaigns. She has been used in various ways and although not as versatile as Gemma ward she can still portray a range of looks within photographs.