Finding suitable icons displayable on MATE's panel

[Richard Owlett]

I'm running Debian 12.8 with MATE 1.26.0
I wish to launch applications by clicking on an icon on the MATE panel.

Questions/Problems:
   1. What icons are displayable on a MATE panel (i.e. specs)?
   2. How do I search for appropriate icon?
      The hierarchy under /usr/share/icons has inappropriate structure,
      giving no hint of displayable or not. (color/b&w etc. irrelevant)

TIA