Yes, by using the Lightbox feature or by adding a photo to your favorites. To create a new lightbox, simply click on the link on the sidebar, or in MyPhotocase. A window will pop up showing your current (if any) lightboxes. You can modify existing lightboxes here, or you can create a new lightbox by clicking Create a Lightbox.
The other method for "bookmarking" is, if you find a photo that you really, really like then click the "I like it" button with the cute little heart next to it on the photo detail page. This will add the photo to your My Favorites list which you can access anytime you like.
To learn more about Lightboxes, click here.
To learn more about Favorites, click here.