If you make professional use of your photo equipment it is probably best if you have specific insurance. If your photography is amateur the simplest and much the cheapest way is to extend your household insurance.
If you want specific photo insurance I'd look first for a Greek insurer or a multinational with a Greek office. Even though many of the major insurance companies are multinationals they still tend to operate on a national basis, and in many cases the companies offering photographic equipment insurance are far too small to operate internationally.
I'm sure you will be able to buy a policy that enables you to have cover overseas. At least you can in the UK. The cost of the policy may well vary with where you want to have cover and how much time you spend overseas.
If photojournalists are freelance they will arrange their own insurance as I'm describing. If employed their employer will do it , often as part of a comprehensive business policy which includes cover for employees whilst on company business anywhere.
Some companies offer cover for a total sum insured but most in my experience want a list of what you wish to have covered. It varies.
You should be wary of:-
a) High minimum sum insured /minimum price.
b) The possibility of high excesses being levied if you claim. This is a favourite trick which keeps prices down and gives an often mistaken impression of good value.
c) Restrictive clauses which disallow or place a limit on certain types of loss. Look out in particular for restrictions on claims for theft from an unattended vehicle- especially at night. My own policy does not cover me for theft from a car after 9pm- but then I'm nearly always in a hotel by then. Another favourite is to place a high excess/limit on claims for loss of unattended baggage or where there are no obvious signs of theft. All companies vary on these dimensions. It is worth reading a policy carefully before you sign up.