Take the agreement to a local employment attorney for review. Perhaps there is some loophole that will allow you to display your work without risking suit from your former employer.
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You raise a valid concern. To the extent you agreed that your work product was the property of your former employer, using those materials may expose you to liability. If you are on good terms with your former employer, perhaps you an get their written authorization to use your design materials in your promotional materials. Be sure to consult your own attorney to protect your legal rights.
This sounds like a trademark / copyright issue. You may have a right to "fair use" if you are not making money off the designs, but you would need to speak with an intellectual property lawyer.
You probably have no ownership right to the materials you created on your employer's dime.
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Ask your former employer that fired you how likely it is. Ask us how legal it is. These are not YOUR projects, your designs, or your property. They are works for hire since you were an employee, which means the employer is the author and owns them, not you. Posting them would be copyright infringement. Is it "fair use" to post them to divert business to you, not hardly. If you already posted these, delete them. If you did not post them yet, do not post them. Either way, if you are starting a new design business, then design your budget so as to include a consultation with an IP attorney so you get your designs in order and protected and so you don't design your way into legal trouble.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
I think Bruce is right. The reason that the provision is in your agreement is to address this very type of situation. They do not want you using "their" projects and holding them out as your own. The fact that they fired you (as opposed to a lay-off) suggests that they are probably unlikely to agree. I have not seen the exact language, but you might be able to contact former customers and ask if you can come photograph your place. Then do not call it "your" work, just indicate that they are photographs of projects you worked on.
Disclaimer: This is not legal advice and you should consult an attorney to discuss the particulars of your case.