It's recommended to use the templates but it's not mandatory.

No it's not mandatory.

Any Image file format like png, jpg, jpeg etc..

You can resize the logo by dragging the black dot at the buttom of the logo.

It's not possible to add more than one logo from ESCWA ePub, you need to merge them into one image by using any Photo editor and add it to your cover by using the Existing Logo feature.

For better compatibility with different ePub readers it's recommended to validate your file with ePub checker.

In order to validate your ePub, first you need to download one of the ePub checkers and validate your file by choosing the file or drag and drop.

1) Missing the book cover inside them, it should be on the first "page" of the book. If missing, your title page will become the cover when viewed in the reader's library shelf.

2) Using embedded non-standard fonts.

3) Fixed layout pages, including 2-page layouts. You can't read these on a cell phone screen because your fixed layout gets shrunk to fit.

4) Though your self-created epub may pass EpubCheck, you should test it in Adobe Digital Editions and on your smartphone and tablet Epub-reading apps.

5) Not critical, but doesn't look as professional, missing or inaccurate metadata in your Epub such as Book Title, Author Name (shows as "Unknown author") and short book descriptions. This is all data used by ereaders for sorting and search results.

Distribution to dozens of retail outlets requires compatibility on many devices and ereader apps. Text should "flow" and be re-sizable by your customer as well as able to conform to the "Night Reading" setting (white text on a black page). Also test if you can search for a word in your epub and highlight and add notes to the text on an ereader app.

To unzip ePub file, first you need to change the file extension from '.epub' to '.zip' and compress it by using software like winzip or you cna just use the extract option of Microsoft Windows.