This is usually moiré, caused when fine repeating detail in the photo gets reduced to about 1 pixel during scaling. Websites and phones often downsample large images for display, and repetitive patterns like fabric, tiles, bricks, or screens can create these false line artifacts.

The usual fix is to apply a slight blur before downscaling, so the detail that can’t be reproduced at the smaller size is removed cleanly instead of turning into patterns. A practical rule from the community answer: if you scale down by about X times, use roughly X pixels of blur radius before resizing.

So the workflow is:

1. Make a copy of the image.
2. Apply a small blur.
3. Resize to the intended display size.
4. Export that version for web/mobile.

You generally can’t keep all full-resolution detail when showing the image much smaller, but pre-blurring before resizing preserves perceived quality better and avoids the ugly artifacts. The issue is not the JPEG size itself; it’s the scaling of fine repetitive detail.