Yes, the original title screens are 640x480 images, and JJ2+'s are 800x600.

Simply "displayed in their original resolution" doesn't feel like a good solution, because then if you were playing in anything smaller than 640x480 or 800x600, parts of the title screen would be cut off at the sides. Maybe if they were displayed at their original aspect ratio, but resized so that they take up the full space of the window on at least one dimension? For example, if you were playing at 800x450, the title screen could be drawn at 600x450--the proper 4:3 aspect ratio--but with black bars on the sides?