When you import your photos into a frame, you can adapt the photos to the size of the frame by using:
The Image Frame contextual menu (Adjust Image to Frame). With this option the image cannot be dragged inside the frame. Free scaling has to be enabled in the Image tab of the PP.
The Scale To Frame Size option of the Image tab of the PP. It does the same as Adjust Image to Frame and automatically keeps proportions.
The X-Scale and Y-Scale values of the same tab when free scaling is enabled. Notice that when using Free Scaling, the image doesn't move within the frame. The top left corner remains at its position.
What you should absolutely remember is that any image resizing into the frame will change the image resolution. The best method is to always prepare the photo at the right size in a photo editing program. This is particularly true if your picture needs to be enlarged. If your resolution is too small, the printed image will be ugly and if it's too large Scribus...