$custom_img only exists if you create in the code!

You will have to edit member.php, the section that applies to the "getinfo" template.

(1) You will have the add the field name of the image url to the database query. (I don't know what you guys have called it)
(2) You have to assign the url to the $custom_img variable BEFORE the getinfo template is called then it will be available to you.

JimF will know how to do this so prod him to give you exact code to use if you do not know how.