Welcome to the Quilting Board!
Personally , I think this particular panel would not benefit from attic windows. If I wanted a window effect, I would make a thin window pane with appliqué so as not to block the beautiful scenery of the panel.
Like I said in your other post, no way would I do anything to that gorgeous panel--- except quilt it up and use it for a wall hanging.
This life is the only one you get - enjoy it before you lose it.
I am so sorry that I don't have the knowledge to help you "import" a picture of your panel into some program and then draw lines on it. I know there are many members who do, but they must not be reading this particular posting. I miss alot of items that are interesting but because of the forum layout, there isn't something that draws my attention. I do so hope you can find someone to help you with this panel. Personally I think that it can be spectacular in an attic window setting, if it is done correctly.
You can frame the whole panel with an attic window, but it is probably more effective to cut up the panel. Remember, windows are rarely square. So, for example, if the usable part of the panel is 24x40, you might want to make the ACTUAL size of the individual window panes 8x10. 36x40 could be 9x10 panes. Whatever the size of the panel, you will need to devide it evenly in each direction. If you are not sure how it will be cut up, begin by laying strips of fabric across the panel in each direction to simulate the cuts.