For English songs: the iTunes way
Go to "iTunes Store" on the sidebar, type the song name into the search bar and search it up! (no need for google!) You will need to note down the artist and the album name.
Make sure your songs are correctly labelled by "album" and "artist"
Note: if "album artist" is different to "artist", it will not work! However, leaving "album artist" blank is also fine. To check this, right click your song and select "get info". Alternatively, you can just add the "ablum artist" into a column category by right clicking anywhere on the bar which has "name, artist, date added...etc"
Right click on your song, and select "Get Album Artwork" and voila! your album art will pop up! You're done!
Tip 1: You can batch select/edit songs by holding down "ctrl" on windows and "command" on mac's while selecting songs. Alternatively, if the songs you want to edit are all in a row, just hold "shift" and press down/up.
Tip 2: If your music is being organised by Artist and you change the artist name, it may vanish when you confirm, as it will jump to it's new alphabetical location! Make sure your music is organised NOT by what you are editing!
Album Art for songs not in iTunes?
I know many of you listen to asian songs or perhaps indie songs as well. If you want to find album art for these songs, you'll have to do it manually. Here's how:
Obtain the album art somehow using a google search. Here are some nice album art searches for chinese songs:
Go to iTunes and right click your song. Select "Get Info" and select the very last tab on the right called "Artwork". Click "Add...", locate your album art photo, and you're done!
Tip: you can also batch add album artworks for songs in the same album by selecting all of them, right-click>Get Info> Double click on the "Artwork" rectangle.
Note: for this method, no correct labelling is required, although it's in your best interest as iTunes sorts your music using the music labels.
Save this for when you've got a free day to yourself at home! At the end of the day, you should get something like this!