How do I make a photo box

When insert my photos into theme preset photo boxes my photos don’t fit the proportions of the box and get cropped. Part of the photo cannot be seen. I see how one can make a text box in Keynote, but how can one make a photo box that is different then standard dimensions within the themes? Or is it possible to change the proportions of the ones already created within the themes?

Thank you!

Pasquale M.