Big picture in first post excerpt

hi,
my problem is:
I don’t know how to get the first picture of my first excerpt big (and basically how do I get this different layout shown in the picture below)

Here is a picture of what it looks like right now:
Layout before

and here is one how I want it to be:
Layout how it’s supposed to be

So I basically want to keep the posts on my home-page as excerpts but the first post should be different like in the second picture.

(The thumbnails are pictures I cropped in Photoshop and uploaded to wordpress, which means they are not automatically cropped by wordpress. That’s the easiest way for me and it should stay that way.)

I already got a special class for the first post (so for example I could make the background blue for this first post if I wanted to) but I have no idea how to get the first big picture from the first post into my excerpt(1st post).

(The big picture I want in my excerpt on my homepage has the same size as the picture in the post btw.)

Alright that sounds really confusing…
I hope someone can help me anyways.

Thanks!