image moves when on different screen

I've centered my splash page image on my laptop but on a different screen, say an iMac, it's closer to the bottom. I've had a look at the other questions that ask similar things, that say that it's not truly centred and I've tried to correct it but I don't think it worked. Is there any way of getting around this?

Also, when I made my thumbnails hover states, I put a background image in that was the same size as the first image, at least I think it was, but when it loads, you can see probably 10 pixels of the background image around each edge of the foreground image. Is there some quick code that can resolve this?

Thanks so much in advance. Cargo Collective is the best.