Originally Posted by jhsrennie
An epub file is actually a .zip archive. If (in Windows) you rename something.epub to something.zip then double click it you'll see it open just like a zip file. In the list of contents you'll see lots of .html files - you usually get one for each chapter - and also a file called cover.jpg that is the cover image.
So when you back up the epub you're actually backing up everything to do with the book - cover included.
When you import a book to Aldiko it extracts the cover, and also creates an info file called metadata.opf. However I think Aldiko only does this for convenience. Eveerything you need is in the .epub file so that's all you need to back up.
Just decided to back up all my ebooks as only the epubs as you suggested.
Then added a few of them to Aldiko. But now I find they have lost the covers that they had before and my Aldiko looks very messy. Tried to upload a screenshot but it was PNG and would not go.