I have 5760×3840 size JPEG image and when I upload it to a website or try to view on mobile, some kind of line things seem to be going on it. It looks ugly. I tried to resize the image to 1200X800 and it looks fine, but its quality has gone. How to fix these issue without loosing quality?
The moiré effect is due to spatial frequency folding. If a repetitive pattern (cloth, floor tiles, roof tiles...) in your picture is going to be about 1px after scaling, you get moiré. The best cure is to blur the image before scaling it down, to remove those kind of details, which won't be visible in the final image anyway. Roughly, the blur radius should be X pixels if you downscale X times. In the picture below, the moiré on the wall was avoided by blurring. Notice that this didn't impact the final detail on the statue: