So, can now answer your question, Hans-Karl, though am not at home (where the computer there has more capacity than my laptop for travel). This worked at home so hope it works for you.
Step one - select the map and open it as a zoomable map, the purpose it to have the address of the zoomable map (as memory serves, it is best not to zoom & simply open the map).
So, take, for example: Die Schlacht von Laon den 9ten März 1814. Gez.von C[arl] Brügner. Gest.von H[einrich] Brose. Gedr.von Prêtre. [Berlin, 1824].
The main listing is at:
The zoomable map (clicking on "download") goes to:
Step two - start Firefox (the next steps seems not to work in Windows explorer); if you need Firefox go to:
Step three - in Firefox open the dezoomify site:
Step four - copy the map address into dezoomify
That means the zoomable map address or, from above:
Step five - hit the button and wait
The sections are "stitched" together and a percentage complete is shown until all are done.
Note: for very large maps the program would stop from time to time (freeze). By starting over (this is on my main home computer), it worked out. Once tried a few on my laptop and many frooze.
Step six - right click on the map (and it may take some time) until a menu shows where the map can be "saved as" (Grafik speichern unter . . ); normally it appears only possible to save as a png file though one can rename it. It reads:
Downloading takes some time (on my laptop there were some maps which were not completely saved - my main computer showed no difficulty).
Step six - open the map (as a png file) in Microsoft Office Picture Manager (tried some others, though not all, this worked the best).
Due to the size, this may take some time.
Step seven - Export the file as a jpg (or whatever other format) desired (and whatever size - mine are kept at the original size to retain the detail).
This exporting step is used as the png files are very large and slow down even my main computer. A jpg file is easier to work with on a daily basis.
By the way, from time to time have come across other versions on the internet. Often those can be downloaded in full with little difficulty.
The details of the offering by the Technischen Universität Darmstadt are just great.
Believe some have contacted them for permission to use their maps commercially with little difficulty, though do not know the details actually.
All the best - R