I grab all my Artwork with a PC program called SkraperUI, its similar to ScreenScaper software,
Put all your rom's in their right directories, on your PC,  you can also have "folders" in the rom directories to have the Hack's inside that if you want
It would look like this

roms/nes/Hacks.  instead of roms/nesh.  So the Hack Folder would be inside the nes folder,
If you don't want that then you would remane "nesh" to "nes",  As "nes" is a recognised System but add the "h" and not recognised,
Delete any gamelist.xml's

So once you have everything in order now go the SkraperUi and follow along its easy, BUT hang on its not over
Once all scrapped, a small amaount of edit is needed and its easy
SkaperUi will make a ES compatible gamelist.xml but it will have things in a "media" folder (images, marquee, videos) but we want "boxart, wheel, snap,"
So rename those Folders thats inside the "image" folder:  images,  marquee, and video's to:   boxart, wheel, snap,   then drag the folders out of the "media" folder and straight into the roms/nes folder
Open the gamelist.xml with Notepad++ ,  and do a "Find & Replace" for these 3 lines:
image/images  to boxart
image/marquee  to  wheel
image/videos  to  snap

It will run down the list making the changes,

Save.!!!  and now you have a fulling  Compatible ES and AM gamelits.xml  (its AM- Atractt Mode that uses those names for the artwork folders)  with what either artwork you have choosen,  Now you can re-name any Folders back to thier original names ie:  nes back to  nesh  as the gamelist.xml does care what the main folder its in is called, as long everthing inside  the folder matches, only reason now to name it to nesh as its a recognised  Retropie / RetroArch name, not like before with Skraper as it doesn't understand the "h"

Its all about name conventions with the software your using, get that right and everything falls into place

My boxart is "box and game play" picture, then no need for snaps / videos if the boxart shows the game play and gives you more room now on the card

Another thing is you will have to re-name a couple of rom folders, to SkaperUi's naming convention's….. eg:  Technos to nes,  Sufami to snes, Zmachine to zxspectrum, and like i said above nesh-nes, ggh-gg, and so on

Game ext's can play a bit part too, games with the  .7z ext wont be found,  but the same game with .zip ext will be