If I’ve understood correctly @myrmyxo, this reference image (which could be a high-res image or photograph, unrelated to pixel art) should be somehow associated to the current project — i.e., reopening the project should automatically reload the reference image(s) too.
I agree that this is a rather useful feature, especially in the sketching stage, and that it offers potential developments (e.g. allowing a preview overlaying the pixel art work image over the references image as if they were the same size, i.e. with big pixels over the high res image, using it like a tracing board of sorts).
Should the reference image(s) be included into the PMNG project file, or should the project just point to external images? The former would allow projects files to contain any reference images (photos, scanned sketches) in a single standalone file, with a size-overhead penalty (although compression might help a little, but not much for image formats that are already compressed). The latter might be problematic when passing PMNG projects to others, for PM won’t be able to locate the ref images at startup and would have to handle the error somehow.
Also, there’s the question of images paths, i.e. relative vs absolute, to keep into account.
As I said, I think this is a must-have feature for all pixel-artists who like to work with a reference image — be it a photograph or a pencil sketch. For example, I own a Inkling device by Wacom, which allows to sketch on paper with a special pen that records every stroke, allowing to later upload the sketch to the PC as an image (vector or raster); I often use these sketches as a reference for my digital artwork, for I work better with pen and pencil than the tablet (never got used to looking at screen instead of the drawing hand).