Sorry that I am posting so late...but this is what I got out of the piece.
I just read Mister's analysis and I think I agree with it, but I'm going to write what I thought it was before I read the other one. I wasn't able to see the picture the piece was based on and I'm sure that it would also explain a lot more.
I got that the overall form was A B A' C D C' A'', which kind of reminded me of a poem structure.
The A section is from mm. 1-15, with subsections a, a' and a''. a is mm. 1-5 in G ; a' is mm. 6-12 in C#; and a'' is mm. 13-16 in C.
The B section is mm. 16-21, with two subsections b and b'. b is mm. 16-19 in B; b' from mm. 20-21 in Eb.
Section A' is from mm. 22-27, it is in G. It seems to me as a transition into the C section, which is from mm. 28-46 and is in C. This ties back to the A section. mm. 42-46 are a transition in the key change at mm. 47.
Section D would be the key change from mm. 47 to mm. 69 with a pedal G# throughout. At mm. 70 we are back in the C section and back in C. Section C last until mm. 83.
mm. 84-89 are a restatement of the A section, but this time in C, which seems to be the pedal this whole piece was mainly built around.
Again I think I like Mister's analysis, making the key change the pivot of the piece and not as complex, but I thought I would be true to the actual work that I did....