Nikos, basically, that's it, but...
The short version is that public domain basically means open ownership. So, at least in the United States (I'll explain in a second), each of us "owns the rights," and can do what we want.
Outside the United States, the same probably applies, but check your local laws to make sure that your local laws recognize the idea of a public domain and that the copyright laws don't magically (and stupidly) apply to foreigners. I think Greece is smarter than that, but one never knows what anybody's politicians have been bullied into passing in the name of globalization.
Also, when it's your own neck on the line, always check the copyrights for yourself anyway. If it's here, we probably checked the rights, but a lot got grandfathered in (as in, "everybody else calls it public domain") and something can sneak through as a part of the comic without our noticing. Plus, if someone mistakenly sues you, your own research will carry much more weight than "some guys on the Internet said it'd be OK." And if you're wrong, your research will limit the damages.
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