You can control the resizing of each image through its Crop settings.
To reach the Crop settings, click the image> Go to the Element settings (first row), Hit the circle icon> and choose Crop.
In the Crop settings, there are two main options:
1) Contain - will make sure that the image will always be fully displayed.
2) Cover - will make sure that the image will cover the entire stripe.
Using the Crop settings, you can also manual scale the image to the required size.
With horizontal images, it's almost always better to reduce their size, as mobile platforms are usually being used vertically.