"Format Background" Picture turns out blurry when running Preview function.

I've reviewed similar discussions on the site, but haven't managed to clear my issue, I apologize in advance if this issue has been presented before. 

 

I'm working on a Storyline project with the preset slide size of 16:9 (960x540 px).

 

I click on "format background" and import some images that I designed on Photoshop. The pixel dimensions of said images are 16:9 (1920x1080 px), thinking that sampling down doesn't affect the quality of my images as would sampling up, right? And indeed, on the slide view, my background images look nice and clear. 

 

There's something that worries me, that is, that when I click on the Preview buttton. My images are stretched out and thus look blurry and grainy. It's not until I click on the "Zoom to fit" button on my Player options that it samples down to my original high quality picture background. 

 

Is there a way to work around this issue? is there a better pixel dimension I should use, can the "Zoom to fit" function be instilled automatically when starting my course, can I make the course not stretch out my project when clicking Preview? Is this only a problem right now when previewing my project but it won't be an issue when I publish my project and set the image quality export settings to HIGH?

 

I appreciate any advice, kind regards!