There used to be a feature in 1Password for Windows which enabled me to save (to my hard disk) a custom icon.
I noticed it seems to have disappeared in the latest version because I went to add a new entry and found I was unable to download the custom icon.
I read the release notes and there wasn't any mention of it being removed.
1Password Version: 7.4.758
Extension Version: 4.7.5
OS Version: 1909 (18363.720)
Sync Type: Membership