Since (from what I understand) the issue only occurs to imported posts I suggest you try the plugin Search and Replace. It’s a quick and easy way to solve this problem.
WP stores the uploaded images in your database. Imported images probably direct to the old root, so you should change that to the new root. Search for the string '/wp-content/uploads' and replace all with '/wp/wp-content/uploads'. Be really careful with this plugin though, it’s a powerful one.