I think you may be talking about the reference images, but let me know if I'm on the wrong track here.
ArtRage full version allows you to load reference images which appear pinned to your canvas and can be moved, scaled, rotated and so on for you to work from. An example image is here http://www.artrage.com/artragefeatures.html under "Tracing Images and References".
References images can be opened by pressing ctrl+R or by choosing "Open a reference image" from the Tools menu. You can load a whole bunch of these at the same time and position them around your painting for you to work from, and you can also right click any reference image to make it a tracing paper under your current work.