Your "fair use" claim was weakened when you moved from using the image in a school project to using it to promote your abilities to potential employers. Yes, it's the same piece of sample advertising, but the notion of "fair use" focuses on "use." If you change the use of the image, then you change the potential applicability of the fair use doctrine.
You should contact a lawyer experienced with copyright issues to help you figure out what you should do. It may be that you have no liability at all, and it may be that your liability is far less than what is demanded. Only a lawyer can help you understand this and prepare an appropriate strategy.
I know that I and many other lawyers at AVVO would be willing to try to help you. One thing is for sure: ignoring the letter is probably your worst strategy.
If you did not purchase the image and obtain a license for its use or the use is other than licensed you have an infringement issue.
I agree you need to consult with an attorney that specializes in copyright issues so that you can respond to the letter you received appropriately. This is not, IMO, a DIY project.
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I agree with my colleague that it's the nature of the use that's what you'd look at in analyzing if your portfolio is a "fair use" of the image or not. Since the most important fair use factor is the allegedly infringer's impact on the market of the work, I agree that your promotional use, _after_ your academic use, skews against you and in favor of Getty.
The lawyer to consult is Oscar Michelen, who literally wrote the book on Getty Images claims. He knows more about their business and the issues involved than anyone, and you can find him here on Avvo.
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