Assuming you mean inside images (not the cover), KDP still recommends 600 pixels wide by 800 pixels (or whatever suits) tall. That unfortunately will look rather small on the larger, high-rez tablets. I have therefore begun to replace the old images with ones that are 800 pixels in width. I also use a
width="100%" instruction in my html. Hitch, who knows more about this stuff than I do, is strong against the 100% solution, but it works for me.
You can see (borrow / adapt) my style sheet from my blog:
http://notjohnkdp.blogspot.com/