The highlighter opacity setting only applies to text highlight, and has no effect on image/freehand highlight (which appears to be equivalent to "100% opacity" setting (not actually 100% as you can see through it - which is ok).

Also see,
https://acrobat.uservoice.com/forums/590923-acrobat-for-windows-and-mac/suggestions/35128342-image-highlight-freehand-tool-anywhere-on-page
and,
https://acrobat.uservoice.com/forums/590923-acrobat-for-windows-and-mac/suggestions/18931885-freehand-highlighter