alex.alp on "[Plugin: WP First Letter Avatar] Change mystery avatar"

Hello, i want to change the default WP mystery avatar, in the plugin there is an option, but i did not understand the brief explanation, it says "Name of the file used for unknown useames (without extension)", but i added it and it didnt show, can you explain it to me or better yet, add an option to choose the image we want to use from the Library? this way nobody will have this problem again :)

https://wordpress.org/plugins/wp-first-letter-avatar/