Usually old window frames like these need a bit of repair or reinforcement. This is easy with some small nails and wood glue. I would personally avoid gorilla glue, that isn't easy to hide if it drips, since there is no painting going on. I use picture wire and screw eyes to hang them.
This first one I filled with photographs of flowers I've taken. The panes are 8 x 12, so I easily had some photos enlarged for around $2 each.

This second one I cut thin pieces of board for each pane and covered them with wrapping paper. I then stippled some paint of pink, yellow and green lightly onto the flowers. It's so light you can't see it here, but it worked out cute. I made this frame first, it's been hanging in a bathroom for almost 2 years now, so I've decided to have some photos enlarged and replace these pieces of wrapping paper. This one I will fill with photos of cool buildings, instead of flowers. I have the photos chosen, I just need to get them printed.

The panes in this third frame are so long that I might end up doing something different. Yes, I think it would look real cool to have three long cropped photos in here, but I just don't know what theme. I'm currently trying to come up with an idea for it. Recently I saw an issue of Cloth, Paper, Scissors, and there were some neat pieces of artwork that were mostly fabric and stitching. Lots of abstract fun stuff, so I kinda considered that.

If I put photos in this third frame, what should I photograph that would crop nicely long and narrow??? Any suggestions? Plus... with only 3 panes, there needs to be obvious cohesion.
Hmmmmmmm.
There's loads of things you could do with old window frames:
*Put cork in each pane and use them for a family noteboard; different panes for different things. (shopping, errands, the kids sports games, postcards, business cards...etc.)
*Put metal sheets in each and use it for a magnet board in your craft room.
*Make a fun style of French noteboard, padding each section and running ribbon across it diagonally.
*Hang it on a wall and paint designs on the wall inside the panes.
*Use it for a family photo frame if you have the same number of family members as the window has panes.
*Make canvas' to put in each pane and paint still life onto them.
*Quilt mini coordinating squares! (this would be cute in a bedroom)
K, that's ideas for you, now if you have any for what I could photograph for my third window frame, I'm all ears.