That photo of the six of the IF seated on a greenhouse roof is from Tobolsk. I would say late fall, 1917, judging by their clothes. It's not from 1918; the weather hadn't improved that much before they left in late April, and in any case I doubt the new commandant was as lenient as the earlier one (Pankratov?) I believe I read somewhere that Nicholas helped to build the wooden structure that they sat on and/or the stairlike structure that led up to it (see TOM's link).
Pankratov was in power at least until 6/19 February 1918, possibly longer. (That was the date the second regiment demanded his resignation, but I'm not sure when Pankratov actually left.) Other photos of NOTMAA on the greenhouse roof also show snow on the ground, but I suppose that's not much help in Siberia....
I'll check Nicholas's diary for further clues.
Interestingly, as Lisa noted , it's hard to tell who is who. Two of the GDs seem identifiable but the other two could be either of the two others.
Left to right: Olga, NII, Aleksei, Tatiana, Maria, Anastasia.
This closeup, likely from the same day, makes it easier to see who's who:
Seated, left to right: Olga, Anastasia, NII, Aleksei, Tatiana. Maria stands behind.