Here is a recent thread on the issue:
https://www.mobileread.com/forums/sho...d.php?t=265355
It all depends on how the publisher has done the css and html, in some cases you might find it easier to just resize the images themselves with tools such as ImageMagick, in other cases you can just edit the css, but in some cases there might be no option but to go through and edit each individual html tag.
If you could post an example of how a typical image is referenced from the html, along with the relevant css styles, then someone might be able to suggest the easiest way to do it.
Edit: AFAIK there is no way to do SVG just from CSS alone, you need to have a SVG wrapper for each individual image if there isn't one already.