It's on the internet, so it is ok to use it.
FALSE. Simply because an image is found on the internet does not mean that it is in the public domain. Unless the author of the work has explicitly stated that his work is "public domain" or that the copyright has expired because the work is very old, then you must assume it is not. Further, a person who posts an image on the internet and claims that you are free to use it may not have had the right to post the image in the first place. Thus, your use of the image may violate the rights of the actual copyright owner.
It is Fair Use.
USUALLY NOT. Fair use of a work for the purposes of merchandise sale is treated very differently than for informative purposes or for commentary. In general, a claim of fair use of a work when it is used on merchandise may not hold up in court, especially if the merchandise is sold for profit.
I took the photo, so I can use it however I want.
FALSE. Simply taking a photo of a person, company, brand, logo or the like does not afford you the right to sell merchandise featuring that photograph. There are two distinct intellectual property rights in a photograph: (1) the rights in the photograph itself and (2) the rights in the subject of the picture, such as the product or person shown in it. For example, if you take a photo of a car or celebrity, you only own the rights to the photo, but not the right to use the photo of a celebrity for merchandise sale. In order to sell merchandise with the image, you will need to obtain explicit permission from the celebrity.