You should be storing the image's filename in the database. So if you have a table users you could add a column profile_pic, for example. On your server have a directory storing the images (maybe public_html/images/profilepictures).
Then in your PHP code you can combine the directory with the filename to retrieve the image:
// query your DB for $user info
// Output image
<img class="profile-picture" src="<?= PROFILE_PIC_DIR . $user['profile_pic']?>" alt="" />
by EvolutionFallen: Missing closing PHP tag