From Halopedia, the Halo wiki

Uploading avatars

  • To upload an avatar for your profile:
Step 1: Save the selected image to your Computer.
Step 2: Click "Upload" (the link is underneath your pre-avatar image).
Step 3: Click Browse, and select the file of the image you want to upload.
Step 4: Hit OK.
Step 5: Congrats! Enjoy your new avatar.

Note: Please make your avatar appropriate.