emilyTK

Step 1: Make a child theme.
Step 2: Find a plugin.

Lol, it’s questions like these that go into the “Plugins and Hacks” forum. 🙂
Anyways, you’ll want to put them CSS bits into style.css.

You don’t even need to touch functions.php for these kinda things, and I’d recommend you just leave it be unless you are absolutely positive that you know exactly what you’re doing.

Pia, I’d really love you to be a bit more specific if you could. Or, you could ask around in a Danish WordPress forum.
I’m not sure I even understand what you’re trying to do on your website. Nobody here could answer your questions like this.

(Also: Netflix, for one, are willingly paying engineers so that they would break Netflixes services. So you might have done WordPress a favour.^^)

For one, the figure element your images are embedded in is limiting their width.
You should set:

figure[id^="attachment_"] {
	width: inherit !important;
}

And since you’re working in a responsive theme, I’m recommending not to set the images itself to a fixed value like 700px. (Except if you do it in media queries.)
If you want your images to be as wide as your column, you need to set this:

.wp-caption img[class*="wp-image-"] {
	width:100% !important;
}

Alternatively, if that isn’t what you were asking for, try fiddling with inherit on the image, too, instead.

Also: I’d make a case for child themes, since you said you don’t want to mess with the code. Well, child themes are exactly the way to go when you don’t want to mess with the code.

Sweet, alright!

One more thing: Your thumbnail backgrounds depend on your slider. If you’ll choose to deactivate your slider plugin in the future, make sure to save or copy the slider background image in http://www.neddanagel.nl/wp-content/themes/Interstellar-child/images/slider_bg.png, so you can set a new path for your thumbnail titles.

(Edit: I just noticed that you made a child-theme, so you might just be fine. Sorry for the confusion.)

You’re absolutely right: My first response works for your slider only, my second response for your thumbnails.
Let’s worry about what you’re doing later-on later-on. Like I said: It all depends on the context. I need to see the HTML output you will be working with later-on, so that we can figure out a proper solution.
So as soon as you have your “later-on images” set up, just come back to me and show me what you have.

I do.
It would help a lot to know which images you’re relating to in effect, though, since it all depends on the context.

Good luck! 🙂