Select the Default Style you want to work with. That Style will be applied to your selected image.
Note: The Style is not actually applied until you click Apply Changes, so feel free to preview all the Styles until you find the one closest to your desired look.
Previewing a Style
From the bottom of the Designer View, you can navigate through your selected images to preview the selected style.
When working with images with different orientation (e.g. horizontal and vertical images), you can select the Sort option from the bottom of the Designer View page to show All Images, Horizontals, Verticals, or just the selected Images. Whatever option you select, you'll notice the image count will update to reflect the number of images fitting that option in your selection.