I had the same problem initially, so I found an existing ad the size I wanted, copied it into my paint program, then pasted my ad into the space before saving it.
Panos, I'm aware that this is a sensitive issue for people from your part of the world. Can you please explain calmly what the issues are, and what you'd like to see happen in a Private message to me. I'll then discuss it with the other administrators and we'll try to reach an acceptable outcome ...