If you've lost some files in Dropbox, there are a couple of things you can do to try to recover them.
First, every time you preview or publish a version of your book, we save a version of your manuscript files (we don't save images; we only save the files with the book's text).
To find the Versions page for your Leanpub book, go to:
Author > Books > [Your Book Title] > Preview or Publish > Manuscript Versions
Second, you can usually recover deleted files in Dropbox itself.
To do this, log in to Dropbox and go to the folder for your book. You should see a "Show deleted files" option to the right, as you'll see in the bottom right of this screenshot from an example book: