It turned out to be analogue SSTV, Robot 36 mode, over FM centred on 255.560MHz.
It seems to repeat the same image for quite a while. So far it has sent these pictures, received with QSSTV:
The colour looks worse than NTSC :P I'm not sure why it is so bad. Similar reports have been noted here and here (forums.radioreference.com) and on youtube here.