When are ImageSet pictures loaded?

If I have “.png” bitmap pictures present in a folder, I can choose to open those files to import the pictures at runtime. How does this work for the Image objects – ImageSet Pictures? Are they dynamically loaded upon usage, or are they all loaded at runtime? If the former, is there a way to explicitly load an ImageSet picture other than using a reference?

ImageSets are loaded as needed

Or you can open one explicitly as a Picture (see http://documentation.xojo.com/api/graphics/picture.html#picture-open and http://documentation.xojo.com/api/graphics/picture.html#picture-type)
Picture.open on an icns file will give you an “image” type

Or you could open the component images one by one and then create a new image from them ( see http://documentation.xojo.com/api/graphics/picture.html#picture-constructor(width_as_integer-_height_as_integer-_bitmaps()_as_picture) )