CSV files cannot contain images. For this reason, while importing a CSV file from another app, Book Tracker cannot restore your old covers. Book Tracker tries to automatically add missing images whenever possible.
If this is not possible, you can proceed in this way to recover the covers: open the detail of each book and accessing the Edit page you can automatically download all the information of a book by clicking on “Update book online“, including the cover. You can also manually import an image or take a picture of the book cover. This procedure must be carried out for each individual book.