The quality of image is less than its original quality

When uploading an image to place it as an opening screen for the program, its quality is less than its original quality
Is the problem in the image format
or in the dimensions of the image
And what are the dimensions in which an image is made to be the size of the mobile screen and with good quality

Perhaps read about factors affecting image quality in these links

I found the same problem while working on an graphic intensive app (a graphic novel). For image display I tried using the 'canvas', the 'background' and the 'image object' which seemed to be the best quality but not as well as the original. It was only when I used the 'webviewer' when I was able to achieve image quality as well if not exactly as the original.