Fozzy, here are some of my tips on navigating the German-language-only site that I learned through trial and error as I don't read German myself.
On its homepage click on the Iron Cross icon called "Inhalt" on the page bottom. This will give a pop-up page link to an index of various book or article page links on the left-hand side of the page under the title "Zum Geleit".
When accessing a page, there will be a grey bar at the page top: clicking the "Ubersicht" button in the top left-hand corner will return you to the main index. Also, a floating grey bar at the screen bottom called "Zurük" is the "Go Back" button. Many of the documents with uniform plates also show icons of open books at the bottom of the page with arrows on them to navigate back to the previous plate or move onto the next plate without having to return to the book's index.
These are the uniform book links to look for:
Deutsche Uniformen v. H. Knötel d.J. u. M. Lezius [cigarette cards of individual figures] There's a button on the grey bars at the page top for the periods that you want.
Knötels Uniformkunde [The master work of its period on Prussian, German and world uniforms throughout post-gunpowder history in individual and multi-figure plates. You'll have to look through the index for individual plates that match your period.]
Uniformen der Alten Armee (Zigarettenbilder von Waldorf-Astoria) [June 1914 German uniforms: cigarette cards of individual German Navy and Colonial uniforms. Just go directly to the book icon marked "Uniformen" on its index page for the uniforms; the other two links are text only with just a few illustrations. I think that all of these are too late a period for you, it's outside my area of interest so I didn't look at them much, but maybe look at last just in case.]
And in case your friend Kermit is doing the other side, this should help him, although I don't know the quality of the 1870 plates as I didn't bother checking them out.
Les Uniformes de l'Armée française 1690 depuis 1900 par Dr. Lienhart & Humbert [French language. French uniforms from throughout the indicated periods using plates with a combination of lifelike and schematic drawings. Navigate from plate to plate using the "Vorheerige Tafel" and "Nachste Tafel" buttons at the page bottom. The central "Ubersicht" button returns you to the book's index of link pages. It also includes Confederation of the Rhine, Italian and other French allied states. Also note that while the plates themselves are in French, the page links to the individual plates are in German so for the French "Chasseurs ŕ Pied" it will say "Jager zu Fuss" as the link name.]