Basta instalar o "Shell Integration" e ele será instalado automaticamente para você.
No menu iTerm2, basta clicar em "Instalar integração do shell"
Você deve ver a saída indicando que o imgcat está instalado:
$curl -L https://iterm2.com/misc/install_shell_integration_and_utilities.sh | bash
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 2694 100 2694 0 0 2890 0 --:--:-- --:--:-- --:--:-- 2890
Downloading script from https://iterm2.com/misc/bash_startup.in and saving it to /Users/allan/.iterm2_shell_integration.bash...
Checking if /Users/allan/.bash_profile contains iterm2_shell_integration...
Downloading imgcat...
Downloading it2dl...
Adding aliases...
Done.
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The next time you log in, shell integration will be enabled.
You will also have these commands:
imgcat filename
Displays the image inline.
it2dl filename
Downloads the specified file, saving it in your Downloads folder.