At any given time, G'MIC manages a single list of numbered (and optionally named) pixel-based images, entirely stored in computer memory (uncompressed).

The first image in the list has index 0 and is denoted by [0]. The second image in the list is denoted by [1], the third by [2] and so on.

Negative indices are handled periodically: [-1] refers to the last image in the list, [-2] to the penultimate one, etc. Thus, if the list has 4 images, [1] and [-3] both designate the second image in the list.

A named image can also be designated by [name], if name uses the character set [a-zA-Z0-9_] and does not start with a number. Image names can be set or reassigned at any time during the processing pipeline (see command name for this purpose).

G'MIC defines a set of commands and substitution mechanisms to allow the design of complex pipelines and operators managing this list of images, in a very flexible way: You can insert or remove images in the list, rearrange image order, process images (individually or grouped), merge image data together, display and output image files, etc.

Such a pipeline can define a new custom G'MIC command (stored in a user command file), which can then be reused as a regular command within a larger pipeline.