The updated php method I adapted now works in QGIS.
If you get the streaming of images working as per my post then you can add a field to your QGIS layer with the following calculation (new text field):
concat('index.php?pid=[your_project_id]&xmlfid=[your_form_id]&parentkey=', "PARENT_KEY",'&filenm=', "[the_image_field]" )
Then in your form set that field to the type "Attachment" and the default path to be your web folder that you've placed the php files.
Set the Integrated Document Viewer to Web Content and when you 'identify' the point, the form will open and load the image directly from ODK Central.
Note that it doesn't resize! Yet.