It's automatic with the default settings when you transfer a book.
I disabled the on-the-transfer kepub conversion in the settings as I prefer the more accurate page and font layout/rendering of epub.
I have edited the CSS in some really old ebooks so that images are 2/3rds of screen width.
Assuming an outer default centres the image and something outer has height: auto;
Code:
.figscale {
width: 1920 px;
max-width: 67%
}
Then the HTML might be
Code:
<div class="center"><img class="figscale" src="../images/00013.gif" alt="" title=""/>
Also in Calibre, select "tablet" as device in conversion settings to avoid image rescaling. Seems to work best with all my DOCX, mobi and AZW conversions to epub.