public TMultiImage = class(TBaseImage);
ImagingClasses (at line 180)
Images are independent on each other and they don't share any common characteristic. Each can have different size, format, and palette. All methods inherited from TBaseImage work only with active image (it could represent mipmap level, animation frame, or whatever). Methods whose names contain word 'Multi' work with all images in array (as well as other methods with obvious names).
TPersistent TBaseImage TMultiImage