Images are the basic entities used by GIMP. Roughly speaking, an “image” corresponds to a single file, such as a TIFF or JPEG file. You can also think of an image as corresponding to a single display window, but this is not quite correct: it is possible to have multiple windows all displaying the same image. It is not possible to have a single window display more than one image, though, or for an image to have no window displaying it.

A GIMP image may be quite a complicated thing. Instead of thinking of it as something like a sheet of paper with a picture on it, think of it as more like a book, whose pages are called “layers”. In addition to a stack of layers, a GIMP image may contain a selection mask, a set of channels, and a set of paths. In fact, GIMP provides a mechanism for attaching arbitrary pieces of data, called “parasites”, to an image.

In GIMP, it is possible to have many images open at the same time. Although large images may use many megabytes of memory, GIMP uses a sophisticated tile-based memory management system that allows GIMP to handle very large images gracefully. There are limits, however, and having more memory available may improve system performance.

If an image is like a book, then a layer is like a page within the book. The simplest images contain only a single layer, and can be treated like single sheets of paper. Sophisticated GIMP users often deal with images containing many layers, even dozens of them. Layers need not be opaque, and they need not cover the entire extent of an image, so when you look at an image's display, you may see more than just the top layer: you may see elements of many layers.

In GIMP, Channels are the smallest units of subdivision in the stack of layers from which the image is constructed. Every Channel in a layer has exactly the same size as the layer to which it belongs, and, consequently consists of the same number of pixels. Every pixel can be regarded as a container which can be filled with a value ranging from 0 to 255.The exact meaning of this value depends on the type of channel, e.g. in the RGB color model the value in the R-channel means the amount of red which is added to the color of the different pixels, in the selection channel, the value denotes how strongly the pixels are selected, and in the alpha channel the values denote how transparent the corresponding pixels are.

Often when modify an image, you only want a part of the image to be affected. The “selection” mechanism makes this possible. Each image has its own selection, which you normally see as a moving dashed line separating the selected parts from the unselected parts (the so-called “marching ants” ). Actually this is a bit misleading: selection in GIMP is graded, not all-or-nothing, and really the selection is represented by a full-fledged grayscale channel. The dashed line that you normally see is simply a contour line at the 50%-selected level. At any time, though, you can visualize the selection channel in all its glorious detail by toggling the QuickMask button.

A large component of learning how to use GIMP effectively is acquiring the art of making good selections—selections that contain exactly what you need and nothing more. Because selection-handling is so centrally important, GIMP provides many tools for doing it: an assortment of selection-making tools, a menu of selection operations, and the ability to switch to Quick Mask mode, in which you can treat the selection channel as though it were a color channel, thereby “painting the selection”.

Indien u vergissingen maakt kunt u die ongedaan maken. Bijna alles wat mogelijk is kunt u ook weer ongedaan maken. Als u op een gegeven moment besluit dat een behoorlijke hoeveelheid van uw recente bewerkingen toch niet het gewenste resultaat opleveren kunt u deze meestal zonder problemen ongedaan maken. GIMP biedt deze mogelijkheid door veel van de bewerkingen die u sinds het opstarten van GIMP uitgevoerd heeft te onthouden. Het onthouden kost echter geheugen en omdat het geheugen beperkt is, is niet altijd alles ongedaan te maken. Sommige bewerkingen kost weinig geheugen om te onthouden zodat veel daarvan onthouden kunnen worden voordat de eerste wegens ruimtegebrek uit het geheugen wordt gewist, andere types bewerkingen kosten echter grotere hoeveelheden geheugen en worden dus eerder gewist. U kunt zelf instellen hoe groot de ruimte is die GIMP mag gebruiken om de bewerkingen te onthouden bij elke figuur, maar in ieder geval is het altijd mogelijk de laatste twee tot drie laatste bewerkingen ongedaan te maken. (De meest belangrijke bewerking die niet ongedaan gemaakt kan worden is het sluiten van de afbeelding; Als u wijzigingen in een afbeelding heeft aangebracht vraagt GIMP u daarom nog eens om een expliciete bevestiging of u de afbeelding af wilt sluiten.)

Many, probably most, of the things that you do to an image in GIMP are done by the GIMP application itself. However, GIMP also makes extensive use of “plug-ins”, which are external programs that interact very closely with GIMP, and are capable of manipulating images and other GIMP objects in very sophisticated ways. Many important plug-ins are bundled with GIMP, but there are also many available by other means. In fact, writing plug-ins (and scripts) is the easiest way for people not on the GIMP development team to add new capabilities to GIMP.

All of the commands in the Filters menu, and a substantial number of commands in other menus, are actually implemented as plug-ins.

In addition to plug-ins, which are programs written in the C language, GIMP can also make use of scripts. The largest number of existing scripts are written in a language called Script-Fu, which is unique to GIMP (for those who care, it is a dialect of the Lisp-like language called Scheme). It is also possible to write GIMP scripts in Python or Perl. These languages are more flexible and powerful than Script-Fu; their disadvantage is that they depend on software that does not automatically come packaged with GIMP, so they are not guaranteed to work correctly in every GIMP installation.