Scanned images depend on lighting, the phone camera, and the actual book itself. You can adjust lighting and try again.
Alternatively, you may find a PDF copy of the book you're trying to import from online tools like 1lib.org or bookshare.org. The easiest way to import a PDF file into Speechify is to upload it through your Google Drive, DropBox, or iCloud.
You may check out this article for more details on how to read PDF files. Make sure that an app for the drive is installed on your phone too.