There is no link from the scanned image on Commons to the text transcription.

In the other direction there are such links.

First, what we call "PR1":

http://de.wikisource.org/wiki/H%C3%B6chstmerckw%C3%BCrdige_Nachricht_von_einem_abscheulichen_Drachen

If you click to the link under [1] (first page of the broad-sheet) you will see:

http://de.wikisource.org/w/index.php?title=H%C3%B6chstmerckw%C3%BCrdige_Nachricht_von_einem_abscheulichen_Drachen&action=edit&image=/wiki/Http://vd17.bibliothek.uni-halle.de/pict/2003/3:634068X/3-634068X_001_X.jpg

Normally you can edit the text but completed texts are not editable for not-registered users.

Second, "PR2" mostly used for whole books:

http://de.wikisource.org/wiki/Aus_dem_Leben_eines_Taugenichts

You can see the program lines how the single pages are added when editing the page:

http://de.wikisource.org/w/index.php?title=Aus_dem_Leben_eines_Taugenichts&action=edit

If you click on the page [3] you will see:

http://de.wikisource.org/wiki/Seite:Aus_dem_Leben_eines_Taugenichts_und_das_Marmorbild.djvu/7

The link "Scan" brings you to the scan.

The Link ^ brings you to the so-called Index-Page where you can see the proofreading status of the single pages of the book:

http://de.wikisource.org/wiki/Index:Aus_dem_Leben_eines_Taugenichts_und_das_Marmorbild.djvu

In better English than mine you can read a help page in the English Wikisource project:
http://en.wikisource.org/wiki/Help:Side_by_side_image_view_for_proofreading

Hope that helps. 

Fascinating, and many thanks for the links and introduction. I had no idea the Page: namespace ("Seite:" on de.) existed. It looks like the support for image-linked pages began around 2006 on the fr. site.

Do you know if the proofreading extension been used for manuscript material in addition to printed works? 

http://de.wikisource.org/wiki/Index:Hexengest%C3%A4ndnisse_1564

http://de.wikisource.org/wiki/Index:Bemerkungen_%C3%BCber_den_Feldzug_gegen_Ru%C3%9Fland_in_den_Jahren_1812_und_1813