Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 80

2.3.3 The Image Window - Your Workspace
The image window is the main window that appears when you open an
image. This is your actual workspace. Although initially the window will show
your image in full size, you can decrease its size so you can use the remaining
space in the window for your palette and tools.
Figure 2.19
The image window is maximized and now also serves as the background window of the program.