At the moment Jitsi accepts it's own (SIP registered account) Avatar image
as "application/vnd.oma.pres-content+xml" document with base64 encoded image.
On the other hand Avatar images for contacts are only accepted as raw PNG data.
Please see PDF below for more details:
IMHO the former (icon as raw PNG image in response to:
is the correct behaviour and should be used in both cases.