I just made my loading screen image 1280x720...

Is that bad? I mean, I want it to be good looking on some of the most common resolutions, but personally, I have a 1080p screen… Will it scale properly for everyone?

Load it in a small webbrowser window and see? It depends on how you embedded the image into the html page. (Unless you just set your loadingurl to the image itself, then I have no idea.)

Uh, I don’t get it. You have 1080p screen but make a 720p image?
Upscaling images usually is a bad idea ( because of quality ), downscaling a 1080p image on the other hand is a more sensible thing to do.

Just set width and height at 100%

<img src="link at your image" width="100%" height="100%"/>

Or use CSS to keep ratio