F-Stop has a functionality named Stacks. This functionality allows the creation of local collections which are stored locally to your computer. This screenshot shows Stacks view
Using stacks you are able to organise photos in a canvas. There are no limits to number of photos storable inside a stack. This functionality is useful if you work on a lot of images and you need to store a future reference to them.
For example, if you are a student and you’re doing a research on specific matter, you can use stacks to make a collection of interesting material including videos.
Stacks Canvas supports multiple levels, resizing of images. Furthermore it’s integrated with slideshow and photo downloading function of F-Stop.
Once a new stack is created, you can add photos to it, simply dragging&dropping images from image browser.