Not sure if this is a bug or maybe a setting I'm missing... I find that depending on the photo dimensions when I replace one photo with another, the picture frame properties are reset to None. I had a file with a collage of 10 photos and had to replace each one and for each one I had to re-choose Scale to Max Fit.
I just did a couple test (both 1.8.x and 1.9 beta) and it seems to work properly if the new photo is the same dimensions or even the same but rotated 90 degrees. But if it's a com