Embedded artwork travels with the file
Some music players can show external cover images from a folder, but embedded artwork is stored inside the audio file. That makes the cover more portable. If you copy the file to another device, the artwork usually comes with it.
Use a clean square image
Most album covers are square. A clear JPG or PNG around 1000 by 1000 pixels is usually enough for modern screens without making the audio file unnecessarily large. Very tiny images can look blurry, while extremely large images can slow down syncing and increase file size.
Keep artwork consistent across an album
Every track from the same album should normally use the same cover. If one song has missing or different artwork, the album can look incomplete in a player. Consistent cover art makes the library feel intentional and easier to scan visually.
When to replace artwork
Replace artwork when it is missing, low resolution, unrelated, or stretched. A good cover should be sharp, correctly cropped, and relevant to the track or release.