First of all, it's not actually a rotated image. That is exactly how it appears in the game.

All CG for any particular VN will be the same size (unless the image pans), which will naturally be a standard monitor resolution. Tilting like this is done to make better use of the space available to show the desired content. For example, if this particular image was not tilted, then they would have either had to show less of the character or make the character smaller. Considering the short amount of time you would normally see it during gameplay anyway, I think it's a decent approach.

Granted, the view angle in this particular image is more extreme than most tilted CGs and this would look much better rotated 90° clockwise. Most VN players should be used to this thus making it more acceptable. There is however a limit and this is pushing that limit quite a bit.