If your former employer's attorney has already concluded that you're in violation of your employment agreement and he felt sufficiently sure to send you a cease and desist letter then you need your own employment attorney to review that letter and respond. No one via this public forum can provide you much, if any, help.
The general rules are that the employer owns the (1) copyright in all creative "designs" drawn by its employees, (2) the copyright in the photographs of whatever it is that embodies those designs and (3) has the right under state law and, if applicable, by employment contract to restrict its employees from disclosing those designs.
You very likely have no leverage in this dispute -- and you may be monetarily liable for any harm caused to your former employer due to your publication of photographs of ITS designs [they are not your designs].
The above response is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
If the company owns the work, which if you were an employee at the time they almost certainly do, you would need their permission to showcase the work unless the agreement allows you to do otherwise.
Unless the state of Georgia has very unusual laws in this area, you are out of luck. When you work for a company, the designs you create belong to the company, not you. That means that when you leave, you cannot use your "portfolio" without permission from the company.
One way to get around this legally may be to provide links to web-sites that show your work---this is because the majority view is that hyperlinking is not copyright infringement (although this situation is another example of why many of us view the majority rule as incorrect---why shouldn't your employers copyright protection extend to unwanted linking?).
I agree with you that this creates a hardship for persons in your situation. Some have argued that the fair use doctrine should be extended to encompass use of copyright works drafted by employees for purposes of seeking alternative employment. However, so far as I know, courts have not gone that far.
Q:"Is it illegal to show creative work I did for a previous employer?"
A: Yes if it belongs to the employer (which it usually does), otherwise not.
Q:"With previous employer for 16 years."
A: One strike against you, as you had access to a lot of trade secrets."
Q:"With previous employer for 16 years
A: That is one strike against you, since if you are there that long you obviously had access to many trade secrets of your previous employer.
Q:"I did sign a confidentiality agreement to not disclose proprietary information."
A: That is two strikes against you, since it clearly shows you knew you had trade secrets and new you were obligated to keep them secret.
Q:"I am a designer so it is imperative that I show samples of work in order to gain employment."
A: That may be true, but it is not imperative and not wise for you to disclose your previous employer's trade secrets. In fact, disclosing substrate secrets is usually unwise when job interviewing as it tells your prospective employer that you are disloyal and cannot be trusted with their trade secrets. In 20 years as a corporate lawyer, I turned down every single candidate that proposed to disclose or use their previous employer's trade secrets. I wanted an employee I could trust not one that was cheating on their previous employer. In fact, it was much more attractive to me when a job applicant said he or she could not ethically disclose trade secrets of a previous employer. Please keep that in mind.
Q:"I do not reveal any proprietary methods, prices, trade secrets, etc... (nor do they really have any)."
A: That is neither believable nor a good thing to say. Your previous employer most certainly did have proprietary information, and for you need to say otherwise can and should turn off any prospective employer to whom you say that. It is much better to say that your previous employer had a lot of trade secrets and you are interested to keep them in you absolutely will not disclose them in order to get a new job.
Q: "I show my portfolio on a private website (only found through a link I provide)
I received a cease and desist order from their attorney"
A: It's just as I thought. You are pretty much screwed at this point, and it is of your own doing. Any prospective employer is likely to check with your previous employer to see whether you are trustworthy and honest. How do you think that is going to work for you now?
Q:"Im torn as to if I should fight it but my future employment depends on this and Im not in a position to take on mounds of legal fees."
A: Absolutely you should not be fighting that fight if you can settle it by agreeing that you will not disclose any trade secrets and removing any thing they say is a trade secret from whatever they want you to remove it from. You might time this negative problem into a positive if you immediately comply and promise never to reveal any secrets ever again. Perhaps that would turn a negative job reference into a neutral or even positive one.
Q:"I did send a reply asking permission from the company but I feel I am doing nothing wrong or damaging to the company."
A: That is a battle you do not want to be waging. You cannot do anything but lose in the short term. You need to settle it so you can move on and not have your past employer killing any job chances you might have for the future. You can be pretty sure they are going to try and kill your job chances if they think you are going to be using their trade secrets for any new job. You need to turn this around. The best way is to hire an intellectual property law attorney to negotiate and amicable solution for you so you can move on and get another job.
Bottom line: What a turnoff to a prospective employer; a job applicant who is fighting with his last employer over alleged misappropriation of trade secrets. Whether you are right or wrong, you need to settle this or you are probably going to be unemployed for a long time. Get a good IP lawyer to help you put this to bed.
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.