EmulationStation has a built in scraper that pulls game box art and description from thegamesdb.
It can be accessed from the start menu in emulationstation, by pressing Start >> SCRAPER.
Scraper Not Saving Manual Edits
If you are having issues with your metadata changes not being saved, you need to select Quit EmulationStation from the quit menu rather than shutdown or restart system. Then your changes will be saved.
Advanced usage and bulk scraper is also available at (Requires unix knowledge) >> https://github.com/retropie/retropie-setup/wiki/scraper