Use px for smaller images and % for images that are in their own paragraph.
Works on old mobi, azw3, kfx and all epub.
em is for fonts, margins and padding. Usually 12pt is exactly 1em and pt works fine for fonts and mostly for margins and padding (some epub renderers seem to use 10pt = 1em for margins/indents, but respect 12pt -1 em for fonts)
Oddly PW3 can be a bit strange on images, but that should be OK there.
Kindle screens are 4:3 approximately and 6″ are 167 dpi or 300 dpi. The 9.7″ screens are 150 dpi and all larger than 6″ on Kindle are 300 dpi. Hence % may work best vs page size, but for smaller images big enough to easily read at 300 dpi, the absolute px size works.
Only one axis should be a %, if used and other set to auto. Enclose in a centred <p ..... </p> paragraph to centre.
Upload epub2 to Amazon KDP, not doc or docx.