Looks like OPNsense is in a transition phase between its old and icons and logos, with the new icon and logo here on the forums and in 25.1, but the old icons and logos still on the OPNsense website.
Now to find a nice, square icon of the new logo for my Homarr setup....
EDIT: seamless update to 25.1, BTW, thanks OPNsense team!
I used this one for my dashboard:
https://github.com/opnsense/core/blob/stable/25.1/src/opnsense/www/themes/opnsense/build/images/icon-logo.svg