I am currently editing a Google spreadsheet that is set up for public editing by logged-in and anonymous users (I am not the owner). I have created a script within the spreadshe
The post you refer to has a clear answer : any call to spreadsheet service will require authorization to work. This implicitly excludes anonymous users. See Zig Mandel answer for explanation on how to solve that (reproduced below)
The button will run under the users permission. If its anonymous it wont run if it needs any api permission (like reading the spreadsheet) since there is no user. For non anonymous users, you need to add a menu item that will trigger the google permission dialog (just do any dumb api call from it). After the user approves it will be able to click the image button