Beneath snow, under snow, or something else
You could say that the city is blanketed in snow.
"Istanbul Snowed Under" is a short and sweet headline.
For a dramatic eye-catching headline, I'd suggest
Istanbul TRAPPED in snow!
For something less sensational any of the the following:
- Istanbul hit by snow
- Istanbul under a layer of snow
For something more literary
- A mantle of snow covers the city of Istanbul
- Istanbul in a mantle of snow
Snowy Istanbul.
Snow-covered Istanbul.
Snow-wrapped Istanbul.
Snow-bound Istanbul.
Snow-entombed Istanbul.
(in increasing order of severity)