Include bitmap in ipe XML file

Hi again, I hope I'm not bothering too much...

Now when I got everything aligned I thought to include background image for reference (which I previously process to extract certain features as marks)
My problem is that it seems I can't include image as file path, but I need to encode it (filter FlateDecode) and hexify then store inside XML file. For that purpose I used zlib module compression, on png image binary data, but encoded content is not the same as content saved from ipe (if I insert same image from ipe GUI and save as ipe XML). Header seems same/similar, but not the stream and as expected ipe can't read my generated file.

Any ideas what might be wrong?

Thanks