In the U.S., the photographer retains the copyright unless it has been transferred to the model in writing or if a "work for hire agreement" has been signed by the photographer. Typical "work for hire" agreements might include staff photographers for a mall photo studio or a photographer hired to shoot production stills for a movie.
Many beginner models mistakenly think images they paid the photographer to shoot belongs to them. A model has control over the use of her likeness but not the use of the photographs. A photographer has control over the use of the photographs but not the use of the model's likeness so both the photographer and the model's authorization are required for most commercial use.
Models typically refer to their images as "my images". When they say that, they are usually referring to their likeness, not the image rights. Photographers also make the mistake of overstating their rights by stating "I own the copyright, I can do whatever I want with the images". While that is typically true of images of a rock, it is not true of images of people, property, or trademarks.
In Canada, the person commissioning and paying for the photos owns the copyright unless it has been assigned in writing.
Generally, if you are the creator of the work, you own the copyright. However, if you create a work in the course of employment, the copyright belongs to your employer unless there is an agreement to the contrary. Similarly, if a person commissions a photograph, portrait, engraving, or print, the person ordering the work for valuable consideration is the first owner of copyright unless there is an agreement to the contrary. The consideration must actually be paid for the copyright to belong to the person commissioning the photograph, portrait, engraving, or print. Also, you may legally transfer your rights to someone else, in which case, that person owns the copyright.