FAQ #81: https://www.deviantart.com/help/81 - How can I create links to other deviants, deviations, or websites?
Image linking
To link images in your journal please follow the code here, and replace the URL Address and Image URL Address with the proper information:
Avatar linking
~username (https://www.deviantart.com/username) will display the member's or group's avatar as a link to their page. Remember, however, that the limit for posting blocks of avatars is sixty four per post, and this will not work in signatures.