How to fix the game pictures (everything that we see in the camera scene) does not change sizewhen change screen resolution?

I am really sorry for my weak English first lol.

I have have made a game and I have a problem. When I change the game screen resolution the picture will change its size and its ugly pixel . My pc is 1366 * 768 resolution and the game picture looks perfect.

example here 1.the resolution that xxxx*768 have the perfect pixel looks.

2.If I change the resolution to 800*600 and 1280*720 and any size that is not xxxx*768, the picture will change its size too.

* If the game resolutions get bigger, the game picture will bigger too.I want to fix it to the original size although the game resolutions are change.