It's a wonderful collection with just a few drawbacks. One drawback is that it's designed for broadband Internet connections: on my dial-up connection at home, I find it very slow sometimes, and occasionally it gets stuck and refuses to function at all.
Another is that it's largely confined to Germany, although it does include some images of objects that are of German origin but currently housed in some major museum elsewhere.
And the vast majority of the photos are black and white rather than color — many of the museums simply scanned and uploaded their already-existing inventory photos of their items, which may have been taken several decades ago. Photo quality is, shall we say, variable.
Yet another drawback is that the website offers an index of all the photos by artist, by subject, by a special image code and by other keywords. This would actually not be a drawback at all, and would be very helpful... if it worked. As it is, it's rather misleading, because the number of things that are thoroughly and properly indexed is rather small, and very few items have all possible keywords entered. So usually only a small fraction of the illustrations you'd really like to see will be displayed if you search on the relevant terms. It's much better at finding things if you already know where in Germany they are located, and in what collection.
Finally, it's a drawback for many of us that everything, including all the instructions, is in German. If you don't speak German, this may leave you flailing about aimlessly. So here are some tips to help you navigate.
Finding items in a known collection
If you know what collection a piece is in, the best way to look for it is to click on Orte (places) on the main page. When the l - o - o - o - o - ng list of places appears, find the one you want -- remember they will be listed by their German names, so Cologne, for instance, is under "K" for "Köln".
(An advantage of searching this way is that the Orte section of the database has a lot more items in it than the other sections.)
Once you've found the right city, most of the time you will want to click on Sammlungen (collections). If instead you click on the name of a place — something like "St. Johans Kirke" (I just made that one up) — it will likely show you pictures of the building, rather than of what's inside it. There are a lot of architecture photos on this site.
When you find the name of the museum or collection you want, click on it to open the folder. If it's a museum of any size, you will find yet more small folders. Embroideries will almost always be under Textil (textiles) but may sometimes be under Kunstgewerbe (which roughly translates as "applied arts"). Church stuff and jewelry (including rosaries) may be under Kunstgewerbe or Schmuck (jewelry). When you get to the lowest category, you will see rows of pictures on the right-hand side of the page, and you can pick what you want to look at more closely by double-clicking on the thumbnail.
On the separate photo window that pops up, there is an item in the menu bar that looks like it says "vergroBen." (It's actually "vergrossen," and means "make it bigger!"). If you click on this, you will get an enlarged view. If you keep clicking on it, usually the second or third click will take you to a place where you can download the picture, either as a JPG file (which is what you've been looking at in the other views) or as a TIFF file, which is a much bigger file but sometimes shows better detail.
Also, if you click on "Dokument," you get a document (duh!!) that gives more details about who made it, where it comes from et cetera.
Finding items by searching
This is more problematic, because the indexing is so far from perfect. You have to remember that, aside from architecture, the focus of this site is on paintings and drawings, and to a lesser extent, sculpture. Everything else may be indexed rather haphazardly, so searches will get some things, but not all of them.
For instance, although I estimate there are a couple of hundred pictures of actual surviving rosaries on the site, dating from the 15th to the 19th century, none of them turn up when you search on "rosenkranz" — German for "rosary". All you get is paintings and the occasional woodcut showing people holding rosaries. (You also get images of Mary Magdalene, for reasons which escape me.)
So. On the main page, on the menu at the top, "Suche" means "search." Click on that. This takes you to a page with three columns of search boxes. "Gesamtindex" is "general index." There are places for you to enter starting and ending dates, if you want. ("Von" = "from," "Zu" = "to".)
Look also for the little round button in the top lines between the words "nur" and "illustrierten." If you click on that (a black dot will appear) you will only get directed to things that have photos online.
You can enter your search word in "Gesamtindex" — this is actually the approach that's worked best for me.
If you have more information and want to try the other boxes, here's what they mean (top to bottom, left to right):
(Aktuere = Catalog [I think])
Künstler = Artist
Beruf = Profession
Werkstatt = Workshop
Auftraggeber = Client
Sonst. person = Other person (anyone else associated with the item)
Sozietat = Society (I don't know what this one means)
(Objekte = Object)
Art des Objekts = Type of object
Material/Technik = Material/technique
Standort = Location
Bauwerk = Building
Sammlung = Collection
Entstehungsort = Point of origin.
The third column is about "iconography," that is, the images and allusions used in the artwork (for instance, a woman pictured holding a small tower might be St. Barbara). Try your search words in the "Thema" or "Oberthema" boxes ("Theme" and "Larger theme" respectively).
When you have filled in boxes and clicked buttons on this page, click on "Suche starten" at the bottom. It will show you totals for each thing you've clicked, and at the bottom, the number of things that meet all your criteria. Click on "Galerie" and it will show you rows of thumbnails that you can click on for a closer view.
You can clear the search screen by clicking on "Suche zurucksetzen".
This should be enough to let you at least flounder around productively in this huge database. I have to admit that I have found a lot of useful things by accident rather than on purpose, but even so, there are really good photos (along with some crappy ones) if you can hunt them down.
Once you've found a photo, BTW, it's fairly easy to direct other people to it. You need the name of the picture file — the string of characters ending in ".JPG" — and then you can merely send people to, for instance:
http://www.bildindex.de/bilder/MI12837a09b.jpg (a very peculiar hat)
http://www.bildindex.de/bilder/zi0690_0009b.jpg (which would make nice needlework patterns)
http://www.bildindex.de/bilder/mi07766b03b.jpg (a picture of St. Margaret — whose dragon made me say "Awwwww, how cuuuuute!")
As you can see, the first part of the Web address is always the same, with just the photo's image name plugged in at the end.