Website pictures showing like negatives

Hi, PavTandon

If this is happening on a specific website, you'll need to reach out to the website owner for more information.

Aside from that, there is an iOS feature called Invert Colors. You may have that enabled:

"Turn on Invert Colors

If you benefit from viewing items against a dark background, you can use Invert Colors to change how content is displayed on your screen. 

To use Invert Colors, open the Settings app, then tap Accessibility > Display & Text Size. Turn on Smart Invert or Classic Invert.

Smart Invert

Smart Invert reverses the colors on the display, except for images, media, and some apps that use dark color styles.

Classic Invert

Classic Invert reverses all colors on the display."

Use display and text size preferences on your iPhone, iPad, and iPod touch - Apple Support

We hope this helps.

Cheers.

Or head off to another one of apples famous hidden duplicate settings: 

settings/accessibility/

per-app settings/Home Screen/ smart invert.