The game covers come from thegamesdb.net which like retropie is created by the community, add on various sources/versions of games and you get more fragmentation and game covers with low resolution etc. You can download jpegs of covers you think look better and crop them to a certain dimension through gimp or something like that and then manually scrape each game to scrape your own images rather than the online game databases.
I know sselph also has a command line scraper that only does images but I think you’d have the same issues with image resolution and whatnot.

As far as the music choice yes I think that would be a lot of work.

Sure thing. So first you create a central folder for all of your images (in can be anywhere) by default the downloaded images from the scraper reside in /home/pi/.emulationstation/downloaded_images/

But in your case it would probably be easiest to just keep the pictures in the same folder as your ROMS for each system (so just in case you need to back things up they are all in the same place) so for a new nes game your images would be in
/home/pi/RetroPie/roms/nes/
Or to keep it simpler you can keep them all in one folder- its up to you.

There are two ways you can add the images to emulation station

Method 1:

You can either add them directly to the gamelist.XML for each system in
/home/pi/.emulationstation/gamelists/

You’d change the existing image file path in the line <image> to your new image file path

Method 2:

After you’ve copied your images to your raspberry pi somewhere, while in emulation station with a keyboard you can go into a system like the nes and then for each game instead of opening it you will press select and there will be an option to choose edit this games metadata.
Select that and go down to image and then enter the new file path to your new image.

Then important part- to save changes you’ll need to choose the option from the menu “quit emulationstation” rather than restart system and that will write your changes. Reboot and they should all be there and you’ll be good to go

The gamelist.XML files are where all of your game data is kept so you can just delete everything you don’t want in each of those files or you can also do that manually by editing the games metadata as mentioned above. Generally speaking the images really are just thumbnails so its probably not that beneficial to have images that at multiple megabytes a piece. Id you want to start with higher quality pics you can take those and run them through an image optimiser online so it keeps a decent quality but doesn’t bog down emulation station. If you get too much going on in emulation station it can cause issues and you have to reallocate ram and stuff but if you optimise your images you should be fine.

[quote=92505]I own Photoshop. It shouldent be a problem:)

I have startet experimenting and hope for the best. Right now i am using a with og 700px. It may be to large but then i can just try a smaller size.

Thanks for all the help:)
[/quote]
I find using a large size image, but reducing the colors makes an acceptable file.
First you only need 72 dpi.
Even 1920 x 1080 at 72 dpi and 256 colors is great for emulator selection pictures.
In Photoshop, choose export for web.
I make a png8 file with 256 or less colors, dither, no transparent background.
A large JPEG may only be 400KB, but decompressed it can use 5-6MB!
A png with 256 or 128 colors at 1080 may be 500KB, but that is neither compressed nor does it need to be scaled up.
I just described using this for 1920×1080 images, but you can do the same for game art.