One of the things I still use in the Calibre editor is the Insert Image tool. If you are inserting a full page image, it gives you this:
Code:
<div style="page-break-before:always; page-break-after:always; page-break-inside:avoid"><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" width="100%" height="100%" viewBox="0 0 1128 671" preserveAspectRatio="xMidYMid meet"><image width="1128" height="671" xlink:href="../birdsargue.jpg"/></svg></div>
With just a click on the image file.
It reads the size numbers from the image and puts them in for you. I don't know how widely this works in terms of all the different readers, but it is fine for my Kobo using kepub books.