1. there are input boxes that load an existing image, or render some vector shapes or text;
2. there are chains of operation boxes that perform things like blur or change opacity of whatever gets fed into them;
3. there are composer boxes that take two inputs and put one over the other, combining them in a certain way (think layer modes: normal, dodge, multiply);
4. there are output boxes that either display the grand result on a screen, or export it to graphics file formats like png or jpeg.