@casablanca You can buy them but you don't really have to. I think once you sign up with an "account"; (which is also free) you get free credits each day. At least that's how it's been working for me. You get even more credits if you look at pictures that have been posted on the site and "like"; them, or "like" the pictures you create using their program. There aren't really any instructions there- so I just started poking around and seeing what was free.
Wow Diana- this one really gave the trees an interesting look!
@casablanca As Ann explained, it is for free. Once you have an account you get 5 free credits every morning. I collected quite a few last month before I started. I used the medium resolution only. One this month is over, I will start to play with the sliders. Good luck, it really is quite addictive.
I love this one, it suits the image so well.
Wow Diana- this one really gave the trees an interesting look!