Problem - Help

Good evening

I have a problem... I've converted my PDF through PDFMate (page to image) to epub. Then, I edited it in Calibre to center the images. My problem is that Kobo does not show the preview of the book. According to them:

"The preview is showing as blank as the content is blank - see attached screenshot. It looks as though your file is just images and you haven't linked them correctly."

If I download the epub, it shows. In my desktop also. The conversion to Kindle also.

Any hints?

Kind regards

JKepler