To be legal, you must get any images you will use from the "copyright owner" and obtain their permission. Most likely you will have to pay for them, although sometimes someone might let you use them for free. You should get that in writing to protect yourself later, to prove you had permission.
The copyright owner is, initially, the photographer. But sometimes the photographer sells the pictures and the copyrights to someone else. It's how some photographers make a living, like paparazzi and stock photographers. And then you must get permission and/or buy them from the new copyright holder.
The best place to buy pictures if you don't know a bunch of photographers, is through one of many "stock photography" places. These companies buy picutures from photographers so that they can resell the rights to other people for use where ever they want.
There are two types of "rights" to use the photos thay you may buy.
The first is just "usage rights", in other words, you pay for the right to use them but the stock company may sell usage rights to hundreds of different people.
The second is "exlusive rights" and this means you own the picuture now and it can not be used by anyone else. Buying the exclusive right is extremely expensive compared to just buying the usage rights.
Then you need to know what "royalty free" images means. It does not mean the photo is free to use ... you still have to buy the rights to use it, but if it's "royalty free" you won't have to pay each and every time you use that image. You may use it as many times as you want in different places, like on your e-zine, your blog and on your facebook, etc. I beleive that most stock companies deal with royalty free images and that is what you want, I would assume.
Here is more information about stock photography:
http://en.wikipedia.org/wiki/Stock_photography
Here are a few of the biggest stock photography companies and image rights can be purchased on line:
Getty:
http://www.gettyimages.com
Corbis:
www.corbisimages.com
Associated Press (for celeb and news images
http://www.ap.org/pages/product/photoservices.html
Here's one where they offer free images ... but I"m sure that they have some way of getting you to pay for images, eventually:
Shutterstock
www.shutterstock.com
There are hundreds of others which you can find by Goggling "stock images" and you'll need to do some research to find out which ones has a lot of the kinds of images you will want to use in your e-zine.
Lastly, you can always hire a photographer and buy the rights to the images that he shoots for you ... that's what the big magazines do.