You can upload a profile photo just for Micro.blog by clicking “Account” on the web. Look for the “Edit” link under your current profile photo. If you’re signed in, click here to choose a new photo.
Until there’s a profile photo set on your account, Micro.blog will fall back on using Gravatar to host and manage your user profile photo. This is based on the email address you used when registering on Micro.blog.