It can be found in your library. In Finder, click on Go and hold down the option key to reveal the Library option. Then you need to go to Containers/Football Manager 2023 v 23 . 1 . 1 + 1759675/Data/Library/Application Support
Then create a 'graphics' folder here and drop your graphics in. (Same principle for 'tactics' and 'shortlists' too if you want to load them in)
I'm on OS 13.0.1 so assuming this is still the case in older versions of Max OSX