I asked on https://github.com/processone/ejabberd/issues/47 and most of popular client doesn't support EXTVAL
xep-0054 is in contradiction of the xep-0153 that say
4.5 XML Syntax
The following rules apply to the XML syntax:
The <PHOTO/> element SHOULD contain a <BINVAL/> child whose XML character data is Base64-encoded data for the avatar image.
The <PHOTO/> element SHOULD NOT contain an <EXTVAL/> that points to a URI for the image file.
The <PHOTO/> element MUST NOT contain the avatar image itself.
The <PHOTO/> element SHOULD contain a <TYPE/> child whose XML character data specifies the content-type of the image data. The XML character data SHOULD be "image/gif", "image/jpeg", or "image/png".
The <PHOTO/> element MUST NOT possess a 'mime-type' attribute.