Here are a part of my code:
First of all in the head tag I have this meta tags:
<meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1.0,user-scalable=1, height = device-height" />
My css hasn't any zoom or similar, and my html is like this:
<img src="/path-to-image-folder-ebook-cover.jpg">
The width of this image is 250px and in photoshop it looks like perfect, but in browser (chrome or firefox is the same) it looks bigger and blured, but, if I zoom down the browser to 80%, the image is perfect