This is very likely a graphics card driver bug, not an issue with Construct 2 or Windows 8. Complain to your graphics card vendor. Up-to-date drivers can still be buggy, and the release of a new OS may mean drivers have been changed but not properly tested. This would also be why the issue does not happen on any other computers.
A possible explanation is the driver is incorrectly putting the 1259px wide image on the next power-of-two size texture at 2048px. This makes it resize to about half as wide. You could test this theory by making it another two pixels taller (129px) and it may again round up to the next power of two size at 256px, and make it display too narrow vertically as well.
Alternatively if you have Catalyst Control Center or the nVidia control panel or any other graphics card tweaking software, try uninstalling it or just reset to factory defaults. These tend to just be crapware that gives you options to break things.