Save this film strip and open it up in PhotoFiltre along with pictures you’d like to add to the strip.
Using the Selection tool on the Rectangle setting, make a selection of one of the white spaces in the film strip.
Then go to your image and press Ctrl-V.
Press Ctrl-Z to undo that.
Then click .
Move the selection over the area you want to add to the film strip.
Press Ctrl-C to copy it.
Go to the film strip and press Ctrl-V to paste the selection and move it into place.
Repeat this for the other spaces.