ebook reader - image scaling ?

Hello all,

In a previous thread I mentioned I found the below ebook-reader :

https://f-droid.org/en/packages/com.github.axet.bookreader/

Currenly I've added an image (by hand) and am trying to get it to stretch to
half a smartphones screen width.   But no matter what I try (and which works
in a HTML page on a PC(1) ) I can't find how to make that happen.

(1) 'width=50%', 'width="50%"', 'style="width:50%;"' and using a CSS file.

Question:
Am I supposed to be able to resize images in an epub(2), and if so how is
that done ?

(2) the above epub reader does have "scale image(s) to fill the screen"
option though, but thats not quite what I was after.

Regards,
Rudy Wieser