I've replaced every single file that had something to do with picture against the original ones that were working.
And finally found that a small png file, that was causing the reboot. That was the slideshow_time_mark (or sth like that)
I experimented that with original width (of 2 px I think) it is working well, but as I change it to something little wider it stops and reboots.
Funny, because in audio_run.xml I created previously new progressbar with different width and heigth, and also the mark is few bucks wider and it works perfectly there.
So, in the end I leaved the picture mark it as it is, but changed only original progressbar graphic and it works as expected.
Actually I have only one matter:
is it possible to compile the OSD with the FW so that it loads faster and does not need any xx.osd.bin file?