How can I make this picture frame looks more like a big window?

I would white wash it

Yes I believe that would help.

Are the window frames in your home also white?

Well if you want it to look like a window, put it on a wall near a window and then try to paint a scene on it that would look like a continuation of what is seen from the nearby window.

Put large outdoor print behind it and dress it with sheer curtains on the front .... Treat it as a window

And place it "any" where you want a faux window ( such as a windowless bathroom)

Put a mirror or mirror tiles behind the frame and hang it opposite a window with a view.

OR, remove some of the small pieces to leave larger panes. Put a print of an outdoor scene behind the whole frame.

OR, add window hardware to imitate the kind of window you envision. ☺️

Leave some space around a few of the photos running along the edges and insert paper the same color as your wall (not white, won't fool the eye enough). You could try inserting pictures only in the center and having the wall color illusion in the spaces around the four edges. If you're going to use ALL the photos spaces then attach some hinges (antiqued looking if that's your thing) and a faux keyhole or window latch.

Put a large mirror behind it. Painting it would depend upon how it would fit in with the rest of your room decor.

definitely white wash it and remove some of the dividers

I have three French doors in my living room. The doors have window panels on each side of double paned glass, attached only on the edges. I carefully used my multi tool and removed the panels. Then gave the glass a good cleaning. Took no time, and I have beautiful open glass doors.

Several good ideas here. I wonder if you have space enough, could you install shutters on either side of it? The color could be whatever will go nicely with the rest of your decor. You could also create a "sill" by adding a narrow shelf. Since it's hanging indoors on a wall, when I say narrow shelf, I mean just a ledge, like an inch wide maybe. To make it even more like a window, build a "case" or frame around it. Real window trim would probably do the the trick. Good luck with whatever you decide.