Professional and amateur photographers tend to have a stockpile of nature photographs that can be put to use. One option is to sell them to a magazine, whether it's a nature, trade or some other type of publication.
Save your photographs as high resolution images. Even for web-based magazines, which tend to post lower resolution images to save bandwidth, high resolution is preferred. The web magazine editor can adjust the resolution at a later date. Note that all print magazines will want high resolution images in at least 300 dots per image (DPI). Also, the larger the photo is in size, the better the quality.
Avoid sending photos that have already been published by another magazine (including web magazines), or other publications. While not all magazines will need first-print rights, some prefer it and others even require it. First-print rights means that your publication of choice is the first organization to print your photograph. Some require exclusive rights to your photos, which means you cannot sell them elsewhere.
Read up on the various types of magazines to see which ones use nature photographs. You'll have the best luck with trade magazines and publications that focus on nature. Each publication has its own set of submission guidelines. Adhere to those in order to increase the likelihood of a published photo.
Query the magazines you have chosen to market your photos. Many will ask that you write an explanation of the photograph, such as when and where it was taken and why it's ideal for their magazine.
Understand that there is stiff competition and that you may not get your photos published right away. Continue submitting photos and queries to various magazines and eventually you will be published.