If your prototype, or your website have low resolution/blurry images, while in Sketch they are much sharper:
For Sketch:
- On Sketch, select the blurry image layer
- Find the Exportable preset panel in Sketch bottom-right
- For Retina - Mark the Exportable asset as @2x
For Vector - Mark the Exportable asset as @1x, SVG - Preview in Browser to view results