What program can I use to add album art to music?

What program can I use to add album art to music?

I have a lot of mp3 files which I get from Game/Anime OSTs, ED/OP Singles
and the Collector's Soundtracks that come with Premium or Limited Edition
Games. A lot of the time these aren't tagged with their album name, track
name and artist (with the exception of some like Disgaea 4, Puela Magi
Madoka Magika).
Now I sync all these files to my old iPhone 4 which I now use as my music
player (because I have an iPhone 5 and my iPod Touch only has 8GB compared
to my iPhone 4 which has 32GB). My computer on which I store all my music,
work and games does not connect to the internet, so it's kind of a pain to
download .txt files with track information and use Windows to change it
manually.
I figure the quickest way to do it is to add album art to all the files
and have them grouped correctly in iTunes. While I can do the grouping
just fine, I can't add the album art.
I am wondering what program can I use to add album art to mp3 files which
would make them still work in iTunes (because if it works in iTunes, it
will work for any Apple product I sync it to).