You may have forgotten about it, but on a Mac / MacBook, there is a profile photo of your Apple ID in the top left of System Preferences (really Apple ID, not just for your Mac).
And you can change it very easily here. It appears in a number of applications, such as iCloud, Messages (where it is possible to set up a separate photo), Contacts, Mail, so do not put something there that could embarrass someone else.
In System Preferences, click on the profile photo (you may have it blank, so there will only be a silhouette). A dialog for finding a photo will appear, unfortunately it is quite an original concept that does not allow you to search and it is also quite slow. So it is ideal to prepare a photo in advance and save it in the Finder so that you can easily find it here. Your Selfie by the way are under Albums -> Selfie .
Then all you have to do is select a photo, look at the preview (you can edit the preview) and save the new photo. The change will also affect all other devices where you are logged in.