Customize thumbnail size
Alfa Ebooks Manager allows to scale the Books View, so you may want to create larger thumbnails than displayed by default. But remember that book thumbnails are stored in the database (not as separate files on your computer) and too large thumb size may slow down the software performance.
You can set the default thumbnail size in Tools -> Options menu. And using the Tools -> Resize Thumbs you can resize all thumbnails automatically. Note that you need to have covers for all books, otherwise the new thumbs will be created from old ones and will have the low quality.