My application is about to get published as a Flatpak on Flathub. It appears in the menu in the right category with the right icon. Plank only shows a blank. Do I need to install a special size of icon in a special location for Plank? Is there something else I need to do?
Plank isn’t “special” - it just uses the information available in the .desktop file to display the icon.
So as long as the icon= value is to a recognised system location then it will be displayed correctly.