To my knowledge --button= is reserved to bottom of dialog but, you can put images in those buttons and keep the image size using --keep-icon-size.
<code> #!/bin/bash
KEY=$RANDOM
imageDir="/home/linux/Documents"
yad --plug="$KEY" --tabnum=1 --form --keep-icon-size --field="!exit!test:BTN" "" \
--field="!/home/linux/Documents/clock.png!Eggs":BTN "bash -c update_apt" \
--field="!$imageDir/eggs.png!Scramble eggs":BTN "xdg-open '
http://yadlive.com'" \
--field="test:FBTN" "" &
yad --plug="$KEY" --tabnum=2 --html --browser --uri="
http://yadlive.com" &
yad --paned --key="$KEY" --orient="horizontal" --keep-icon-size --splitter=200 \
--tab="buttons" --tab="website" --center --width=800 --height=600</code>