Listed below are images cited in the text that I have failed to locate. It does not include citations where I have managed to find something nearly identical (in practice, this means drawings).
Some of these can be resolved fairly easily, by visiting the location and taking a photo or buying a postcard or a guidebook containing the missing image. Others are very elusive indeed, and may no longer exist - after all, 2 World Wars have passed since Wölfflin started to teach.
If you have any of these, I would be very grateful - please scan them as bitmaps or tiff files, rather than jpgs.