Everything works fine with Chrome, Firefox, IE but not with Safari.
If I just put embed-code in the page's html, everything works.
If I try to pass if via form, nothing is shown.
But if I pass code via form, save page that appeared and then reopen it, everything works again!
Thank you for answering, nyx.
print_r shows data as it should be. $_POST['image'] looks to be ok too. I tryed to change encoding, but it didn't help. I guest I will live it as is or use external files and Iframes (everything works ok if data isn't echoed on page directly via request).