Few years since I was there, so I stand to be corrected...
Are you allowed to take pictures?
I wouldn't...
How long is the runway?
2410/2440 metres depending on which one you use
What are the duty free facilities like?
If you're travelling from UK then no duty free, we're all in the EU, however shopping facilities were minimal... from what I remember one small shop
Probably the worst airport I can remember using as a passenger....
Have a look at this for more info..
http://www.hcaa-eleng.gr/thes.htm
Presume you're off to Halkidiki? Beautiful part of Greece...enjoy!