In the default mode images are displayed in a grid with an adjustable number of images per row.

controls

zoom
The number of images in each row can be altered using the slider in the bottom panel, or by holding Ctrl while scrolling over the center view with your mouse.
navigate
You can navigate through the images using the arrow keys (←/→/↑/↓) or by scrolling with your mouse. Press the Home key to scroll to the top of the collection, the End key to scroll to the bottom, and PageUp/PageDown to scroll up/down by a page.
select
You can select the image under the pointer by clicking on its thumbnail or by pressing Enter. A range of images can be selected by clicking on the first image and then Shift+clicking on the last one. Images can be added or removed from a selection by Ctrl+clicking on their thumbnails or by pressing Spacebar.