All applications' desktop entries can be found in /usr/share/applications.
You can create a desktop entry under ~/.local/share/applications to make your own icon.
zzh@ZZHPC:/usr/share/applications$ sudo cp org.flameshot.Flameshot.desktop ~/.local/share/applications zzh@ZZHPC:/usr/share/applications$ cd ~/.local/share/applications zzh@ZZHPC:~/.local/share/applications$ sudo chown zzh:zzh org.flameshot.Flameshot.desktop zzh@ZZHPC:~/.local/share/applications$ mv org.flameshot.Flameshot.desktop ZFlameshot.desktop zzh@ZZHPC:~/.local/share/applications$ vi ZFlameshot.desktop
Replace the value of 'Exec' for [Desktop Entry] with the value of 'Exec' for [Desktop Action Capture].
Delete all the 'Desktop Action' parts, leaving only the [Desktop Entry].