Thank you for your replies!

The file names are alright - I've changed morphos' iso name to m.iso.
I use the following command: "boot usb1/disk@1:,\boot.img bi umsd0:m.iso" - I've tried "boot ud:,\boot.img bi umsd0:m.iso", but after executing it, there was a gray screen with a slashed O at the middle, like this Ø. I've also tried to clear the nvram, it's the same result after it.

Cheers,
Jeno