WordPress Media Library and Image Basics
WordPress Media Library
The WordPress Media Library is a place to upload and store image, video, and documents that can subsequently be used throughout your website.
Adding Files to the Media Library
While media files are added to the library when uploaded from Edit Post or Page, they can also be added, edited, and arranged from the Media Library itself.
To begin visit Media > Add New and upload the files you'd like to be stored in the Media Library. Once the upload is complete visit Media > Library to edit files individually.
After clicking on an individual file you will be on the "Edit Media" page. From here you can change the title of the file, change the permalink and if it's an image you can select "Edit Image" to add a caption, change the size and add alternative text.
Be sure to "Save" or "Update" any changes made the Media Files.
Also note that depending on your browser caching you may need to use the Plugin called Regenerate Thumbnails to ensure that after changing dimensions of an image they resize correctly according to the activated Theme.
For more detailed information on the Media Library please visit the WordPress Codex