This is not related to Bubble or Sendgrid really. It’s a little bit complex to explain but initially, theses pictures was from a gmail account linked to the sender email, Google + account and now, this can come from BIMI specifications. Some mail client (this is actually the mail client that “set” a profile image to email) like microsoft that doesn’t support BIMI actually. For MS, you will need to create a Business profile: https://business.microsoft.com/
Finally, some Mail client will use gravatar account (created with the sender email) to get the profile image/avatar.