I found a super simple way to quickly add an image to an mp3 file:
- Drag and drop the mp3 file into iTunes
- Go find your mp3 file (it’s probably in your Recently Added section)
- Select your mp3
- Hit Command + i (if you’re on a Mac) or right-click and select Get Info
- Click on the Artwork tab along the top
- Click the Add button, select the file you want to use for your image, resize it in the box, and click the OK button
If you’re wanting to use this file somewhere other than iTunes you can drag and drop the file onto your desktop and you’ll see the image is still there showing as the file icon.