I don't think it should stretch, do you mean it doesn't fit properly? Because different devices have different display scaling and aspect ratio, which means banners won't look the same on all devices, so if you have one with a more square shape, it might look weird. In terms of being blurry maybe your downloading a different version if you just use a preview of the image, and you got to realize that being scaled up no matter what will make an image blurry.
Yes, it doesnt fit properly. a lot of pictures tends to get cut off even though they should fit since its to the size range. its confusing, i tried to resize them because i thought maybe then itll fit but it just doesnt. im not sure what im doing wrong or maybe its just because of the laptop im on? I dont really know for sure.
Yeah there are a lot of different display resolutions, a common one is 1920x1080 which I am pretty sure the banner is meant to fit perfectly (not 100% sure on that though). On my device that is currently 1366x912 the banner has less width (or more height) and it would be vice versa for a wider screen. So a banner may look weird on one device, but will look normal on another which is probably the problem. Maybe just temporarily upload the banner and I can tell you how it looks on different resolutions