According to the 0.19.4 developer notes on GitHub:

• default user profile photos will be replaced with an algorithmically generated 'Snoo', known as 'Lemmoo'
• existing users who have not uploaded a custom profile photo will also be assigned a Lemmoo
• Later patches propose to allow users to customise their Lemmoo, including the avatar's colour, headware and other facial features

The change is intended to create stronger brand recognition, inter-instance identity and community.