Czech Resources Online
Google Books has copies of several volumes of the important art journal Volné směry, published by the Mánes Association, although they are erroneously catalogued as different editions of one book. It looks as though volumes 1 (1897), 2 (1898), 4 (1900), 9 (1905), 10 (1906), and 12 (1908) are actually viewable and downloadable PDF files. Several other years are listed as digitized but for some reason not actually available, perhaps due to confusion about these being editions of a book rather than unique volumes of a periodical.
The New York Public Library has digitized holdings of the related cultural and design magazines Žijeme and Magazin dp. These are done as individual JPGs, which makes them easy to put in PowerPoint but not as easy to read through.
I'm somewhat baffled as to just how one easily gets a sense of what digital resources the NYPL has without either having a specific target in mind or else leafing through hundreds and hundreds of images. Searching on "Czech" tells me that there are 2577 images, which is a fine thing, but I really don't want to go through 215 pages of thumbnails filled with things like views of Humpolec. There is, for example, a digitized copy of Nezval's Pantomima with cover by Štyrský, and Biebl's S lodí jež dováží čaj a kávu with cover by Teige. Then there's also the cover of the third issue of the Erotická revue, but apparently only one of the illustrations, a hermaphroditic drawing by Toyen (admittedly one of the better choices artistically speaking, but not exactly representative of the contents). Unlike some people, my patience for going through endless pages of images just to see what's there is rather limited. We'll just be glad for all those digitization projects out there and also for people who sift through them and point out more good stuff than I will ever do here.
Note: Prodded by an Esteemed Reader, I have put in lots of links and also discovered that the NYPL has digitized ReD (Revue Devětsil)--here's volume 2.