You can set the feature image for each Page, Gallery, or Folder on your site. To make this magic happen, navigate to the Page, Gallery, or Folder you want to change. Once there, click Customize > Page Settings (or Gallery Settings/Folder Settings if you're on a Gallery or Folder page).
On the Basics tab, click on Feature Image. This will open the dialog that lets you navigate and choose a featured image from your uploaded photos.
You can remove a featured image by clicking the X next to it in the above menu. This gives you the option to choose a different image to feature.
You can also access the Settings options from the Organizer. Click Organize in your header toolbar, select the gallery/folder/page you want to edit from the navigation tree, then click Settings.
Select the Basics tab and then click the thumbnail next to "Feature Image" to select the photo you want to feature.
Note: If you don't pick a feature image for a passworded gallery, we'll display a lock icon by default.