Remove post thumbnail html attributes

Ok here is the thing.
I need to use custom thumbnail size, so I make it and it’s works fine, but it’s renders something like that

<img width="400" height="300" src="...."

That are correct sizes, but that two HTML attributes (width & height) have some conflict with my css.

So to understand where is the problem, go here https://dajzafree.net/?p=5 and then resize browser, after that use inspect element to remove width and height attributes from img tag, and then resize again.

Hope you will understand.

So my question is, how can I remove this tow attributes (and still have image with that size) or solve that CSS conflict.