How to use images from the internet legally under fair use?

I would be very careful. There is a huge difference between a site which is for small private use and a site which is an internet site visited by many people and which earns income. Some of these images may be copyrighted and others may be trademarked. Some companies will be glad for the publicity and others will sue for infringement. I would start out by taking a selected number and contacting them to ask what their approach is. You might find they want a license fee which then makes it legal and does not subject you to a suit. There is a big different between saying "Coco's new line is too old fashioned" and downloading all of her 2009 dresses for viewing. That is why some print journals have the tag "reprinted with permission."
LEGAL DISCLAIMER
Mr. Sarno is licensed to practice law in NJ and NY. His response here is not legal advice and does not create an attorney/ client relationship. The response is in the form of legal education and is intended to provide general information about the matter in question. Many times the questioner may leave out details which would make the reply unsuitable. Mr. Sarno strongly advises the questioner to confer with an attorney in their own state to acquire more information.