There's a good chance you'll be breaking copyright. And anyway, it's not cool to use other people's work in your newsletter without asking.
There are, however, many authors and photographers who are happy for you to use their work, under certain conditions.
One way to find these content creators is through Creative Commons, a system set up to give writers and artists a standardized way to grant copyright permissions.
For example, many are happy for you to use their content if you give them attribution.
Content creators add a Creative Commons copyright license to their work. The type of license they choose depends on how they are willing for the content to be used. Some will allow it to be used in any way, as long as there is attribution; others don't want their work to be used for any commercial products.
You can search for Creative Commons work online through the Creative Commons website. Then, when you find content that you like, you can check which type of copyright license it carries.
Of course, it relies on trust. But it's a two-way thing. After all, you might like to consider putting a Creative Commons notice on any articles you post online. If all you want is a link, for example, you could gain some web traffic from anyone who chooses to use your content.