WordPress built-in functions to show detail of Author of post, page and site. Here are some function provided by WordPress to get these info.
The Author image comes from the email address set for that user which goes to a corresponding Gravatar. The display name and bio come from the User settings area in the Admin.
Author Detail Code
<div class="author-name"><?php the_author_meta( "display_name" ); ?></div>
<div class="author-bio"><?php the_author_meta( "user_description" ); ?></div>
Note: some of these functions are WordPress 2.8 and newer only.