How to make pages with full bleed / full page images

Hello,

I created a photo book with borderless images (some single pages, some double page spreads) in Lightroom and exported it as PDF. I then imported the PDF into Calibre and converted it to ePub.

However, the images have a thick white border now. The original jpg files in the epub container are 1134x1440px. My book is vertical. I was hoping to get a borderless photo book.

I checked the CSS file and set the margins to zero to no avail. Has anybody a tip for me how to proceed or is this a limitation?

Thanks,
Martin