[Arrandir]

I'm writing a handbook in photography, of course it will have lots of pictures which need high quality.
A draft is now formated in Sigil with HTML and the pictures are in the book. Unfortunately, the pictures had bad quality when I for he first time looked in the book with an ebook reader app. I checked the size of the epub-file which was one fifth of all the pictures together.
Is there a setting to prevent Sigil from compressing the pictures? If not, are there any alternatives?