Thinking a bit more about this, I want to add a clarification (stipulating that solid legal advice is always better than internet legal advice).
The OP has raised the issue of copyright. That seems to imply that he intends to re-sell or otherwise profit from the work he did for his employer. I don't see any way that that will end well for him if that is his intent.
That said, though, if what the OP really wants to know is if he can show his work as examples of the kind of work he is capable of, I would modify my advice.
I've spent all my career working in journalism, public information and public relations. It is common and accepted practice for creatives (writers, photographers, editors, artists, etc.) to show prospective employers or customers samples of the work they have done for other employers or clients.
Having interviewed and hired a great many people over the years, I have always asked to see examples of their work and they always had examples to show. In fact, in the old days, it was common for reporters and photographers to collect a dozen or so "tear sheets" of their best work to save for later use in their portfolios. I've never heard of a publisher ever objecting to this.
If the OP is simply wanting to be able to show samples of his work to prospective employers or clients, I can't imagine anyone objecting. Although, if he wants to post the images on a personal portfolio website, I would recommend getting permission of his employers since that could constitute "publishing" which is different than simply including examples with your resume.
If he intends to profit from the images, that is an entirely different matter and, as other have pointed out, frankly isn't worth the hassle and damage to his reputation, regardless of any legal rights.