On the capture screen, tap the Album thumbnail (bottom left) to open the Photo Library. If you took a photo before opening the Photo Library, you will see your last taken photo. Otherwise, the Photo Library will open with the Album List View.
To get from single photo view to Album Overview, tap Album name (e.g. Recents) and then tap Albums.
The Lightbox is the second album shown in the Album List View.
For a more visual guide, check out our tutorial video (which also talks about the Photo Library starting at 1:53):
You can quickly access albums by swiping from the left margin of the capture screen to the right, independently of whether the Album thumbnail is displayed.
Swiping from the right margin of the capture screen to the left takes you to ProCamera Settings.