I've run across a few books that were excessively bloated to 3-5x the normal epub file size because of uncompressed images. They included images for every book by an author (which I really don't care to see when I'm reading), and multiple cover images. There's sometimes a background image for certain pages... I don't need the faded publishing house name on the publishing page for instance, but if it's a small enough file size, I'll leave it. Once I delete all those types of crap images, I compress the images that are left which usually reduces them to up to 12% of their original size (without quality loss, it can be done very easily), and I'm back to an epub that's under 500KB as it should be.
I never delete images essential to the text, dividers between sections, publishing logos, etc that are really part of the book. But I see no reason to use up space on my reader with images of 30 other books.
I just open those types of books in Sigil and delete the images from there, drag the ones that stay into my graphic program, compress and save, and delete the pages that have all the images on them.