How to fix responsive header?

Hey there, I have a few questions:

1) The image header on the main page seems to behave awfully at different sizes of the browser. It gets cut off at full-screen, but is fine at other sizes…. and then gets cut off again. (See attached images for reference: https://imgur.com/a/0ccmg
) Is there a way to fix this, so that the image stays at full size regardless of the width of the browser window?

2) This is more of a general coding question rather than a specific theme one –
I’ve removed the sidebar of the theme so as to allow the content to span the entire page. But I would like it to only apply to images, while the text spans 70% of the page, maybe using padding around the text? but my coding is shitty at best, so help!