Using others’ work without express permission is classed as ‘Theft’

Who owns the copyright on photographs?
Under law, it is the photographer who will own copyright on any photos he/she has taken.

Using the work of others:
As with all copyright work, you should first obtain permission from the copyright owner before you use someone else’s work. You should also be prepared to pay a fee, as many photographers will charge you for using their work.

Only the copyright owner, (or his/her authorised representative), can give permission, so you should contact the photographer, or his/her company, directly for consent. For images published on the Internet, it is typical to contact the webmaster of the site in the first instance, unless the site provides contact details for the owner of the images.

The copyright owner has no obligation to allow you to use their work, and can refuse permission for any reason. Downloading other peoples work and uploading as you’re own is also illegal. You must always seek permission and approval for use.

Source: CS