Add New Media in WordPress
Add the Media File
In vertical menu on the left, hover over Media and click on Add New. Click on Select Files and navigate to the location of the file you would like to upload. Select it and click Open.
Note: it is recommended that your image be less than 100kb and have an appropriately descriptive name.
Edit the Media File
After the upload is complete, click on Edit next to the file name. In the fields below the image, fill out the Alternative Text and the Description fields.
Alternative Text: Write a complete description of your photo. This field is used by the visually impaired as well as search engine bots so this field should include information about the image and keywords you would like associated with that image and post/page.
Description: Write a complete description of your photo.
For additional information on how images are important to your site's SEO, please visit this Copyblogger article about the new Google Hummingbird search algorithm.