Your question leaves a lot unexplained, such as what OS your computer is using and why you don't connect it to the internet. If you connect to the internet you can import artwork using File > Library > Get Album Artwork.
If you refuse to connect to the internet, or want to use your own custom artwork, iTunes itself allows you to add artwork to songs; select the song in iTunes, choose File > Get Info > Artwork, and paste any image file in the dialog. This won't work if the .mp3 file is locked or encrypted.
If you refuse to use iTunes and are using Windows, there are ID3 editors that allow you to edit ID3 metadata even on encrypted files.
If none of these answers work for you, will will have to provide additional information.