With a recent project, my bitmapped background was larger than the camera size.
I found that I could trim it in Pencil2D, to the exact required dimensions, then make a new frame and place it using paste. It placed it exactly in the correct location.
The upside was that the size of the resulting pclx was reduced from over 4M bytes to 3.8M bytes too.
I always take photographs at the highest resolution, which the camera can. Because you can easily reduce the number of pixels, but it much mote difficult to increase them. Trimming the photograph after initially changing the size in Gimp allows the size to be trimmed to the exact size.
I always have the picture slightly over size, when output from Gimp Because this allows final adjustments of the composition in Pencil2D. This prevents tree branches not appearing to sprout from a character’s ear.