Every time you select an image to use as a texture or for rotoscopy, an image clip and an image source of the selected image is created, regardless of its format (PIC, GIF, JPEG, TIFF, PSD, etc.).
Image Sources
An image source is not really a usable scene element. It is merely a pointer to the original image stored on disk. It is defined as read-only and is listed in your scene in the Sources folder of the Scene Root. It does not have to be reloaded when you re-open your scene. Image sources can be stored within your project, or outside of it.
An image clip is a copy, or instance, of an image source file. Each time you use an image source, an image clip of it is created. You can have as many clips of the same source as you wish. You can then modify the image clip without affecting the original source image.
Clips are useful because they allow you to create different representations of the same texture image (source), such as five different blur levels of the same source image. Also, clips are memory-efficient because the source is only loaded once, regardless of the number of clips created from it.
SOFTIMAGE|XSI v.6.01