In Opacity, you can create documents that contain sets of images, instead of just a single image. This can be useful when you want to design a set of similar or related images or icons. Each image in the document is represented as a frame, which you can add by selecting the Image > Add Frame menu item. You can choose the frame to edit by selecting the one you want in the frame chooser at the top of the document window. You should save your image with the Opacity image format to make sure your frames aren't lost. You can share a layer between frames by selecting the layer and choosing the Layers > Share Layer Across Frames menu item.
Once you have created your frames in your image, there are a few ways you can distribute them. The easiest way is to add a factory and build as the Icon Container format, which will keep all of the frames from your image. Icon Container files can be used in CandyBar (by Panic and The IconFactory).
You can also build your frames as standard format images (TIFF, PNG, PDF, etc.) by adding a factory and setting its Frame to "Folder of All Frames" or "Zip All Frames". This will create a folder or zip file holding an image made from each of your frames.
Finally, you can choose "Preview of All Frames" for the Frame setting to build an image with a protected preview of all of the images from your frames. This is great for making images to use online to promote a set of images you are selling.