Well you'll need to select the link field from your database and put that information into a variable. Then put that variable in your print string that is displaying the <img> tag.
print "<img src=\"$link_field_variable\">";
PHP will read the contents of the $link_field_variable and output that value with the print statement.
If you don't know how to get the data out of the database then searcht the articles on this site for MySQL, and you'll find some good articles.
