@iangilman thanks a lot, then it is exactly what I need! And we have huge images!
One last question: Our multiresolution pyramid is slightly special, because we combine on each level the 2x2 images of the half resolution into a single file. So higher levels have 4x less files than lower levels. Is this a supported format out-of-the-box, or can you give me a pointer how I could add support to OpenSeadragon?