If you click on an image (Slide) in the Slide List & then click Objects & Animation you can adjust the size & position of the image.
You will see in the preview window that the image has grab handles around it.
Just drag these with your mouse to get the appearance you want.
If you are enlarging the image it is best to make sure it has enough pixels to do this, but PTE does a brilliant job of displaying them whatever size.
It is an excellent piece of software, very simple to use but also extremely powerful once you get to grips with it.
Enjoy
Jill