You can customize what happens when you double-click a photo. You can view the photo full-size, open it in a new window, or open it in another program.
You can choose which program to open photos in from the "Edit photos in" popup menu. To open a photo with the chosen application, you can also click its icon in the toolbar or hit ⌘O.
Check "smooth scaling" to anti-alias photos when making them larger or smaller. You can also toggle this preference by choosing View ▶ Display ▶ Smooth Scaling.
Check "enlarge small images" to stretch small images to the maximum size. You can also toggle this preference by choosing View ▶ Display ▶ Enlarge Small Images (⌘^).
You can choose whether to reduce photos to a particular size when emailing them.
You can choose what size to shrink photos to when backing up. You can also choose to only shrink photos rated less than a certan number of stars.
Check "create thumbnails automatically" to create thumbnails for image files that don't have custom icons as you browse. If you turn this option off, you can create thumbnails manually by choosing Photos ▶ Create Thumbnail.
Check "create smooth thumbnails" to anti-alias them. This tends to make photographs look much better, but business graphics may look blurry.
You can choose to automatically go to the next photo when rating a photo by typing a number.
Shoebox normally warns you before moving photos to the trash. You can turn off this warning, but be careful when you hit the Delete key!