thumbnails images

Hello,

I would like ask - how can I change display of picures in Web browser?Because they are displayed little bigger like in creator. 

Images (thumbnails) in preview in creator are good, f. e. 400 x 200 pixels- sharp.(jpeg, quality 100).

 But in browser are displayed little bit bigger. SO they are  unpixelized, unsharp. I need set in Chrome size to 80 percent - then are ok, sharp. 

Thanks

Vlado