I want to use my own images for the 5 slider header images.

These images are just ones I have taken on my iPhone, nothing fancy. I upload to the media section within Sydney, no issues. I do not make any changes or crop, etc.

When I insert images into the header slider area, they appear stretched and way to big. How can I get them either smaller or to remain not as large?

Thank you-
Sara
new wordpress member and new blogger.