When you upload a picture, WordPress stores it in the wp-content/uploads folder of
your site. For example, if you upload a picture named face_photo.jpg to the Magic
Tea House site in January 2014, WordPress stores it at http://magicteahouse.net/
wp-content/uploads/2014/01/face_photo.jpg. Behind the scenes, WordPress also
creates large, medium, and thumbnail-sized copies of your picture with names like
face_photo_300x200.jpg, and stores them in the same folder. That way, WordPress
doesn’t waste bandwidth sending a full-sized picture if a post needs to display just
a tiny thumbnail.
WordPress might store more files than you think. In addition to all the pictures you
insert into posts, all the featured images you use, and any custom header or background
images you add to your theme, WordPress stores files that you attach to your
posts here, too, like PDFs, Word documents, and spreadsheets. Furthermore, if you
change a picture (using the basic cropping, resizing, and rotating tools described on
page 184), WordPress stores a new, separate version of the picture as well.
WordPress calls this repository, which holds your pictures and files, the media library.
To see the current contents of your site’s media library, choose Media→Library
There are two reasons you might want to use the media library: to remove files you
don’t need anymore, and to upload files you want to use in the future.
NOTE WordPress doesn’t let you replace a picture that’s in the media library. Even if you upload a picture
that has the same name as one already in your library, WordPress puts it in a different subfolder or gives it a
slightly different filename. The same thing happens if you edit a picture that’s already in the media library. This
system prevents a number of seriously frustrating problems, but it also means that there’s no way to update the
picture in a post without editing the post.