Download the required artwork from an external website and save it in an accessible place on your computer. A likely source for the artwork can be the band or musician's own website, their record label's website or e-commerce sites such as Amazon, eBay or Play.
Open your iTunes library and locate the music you wish to add artwork to.
Highlight the relevant songs, select "File" in the main iTunes menu and choose "Get Info." A new window entitled "Multiple Item Information" will appear.
Drag the artworks' icon from its location on your computer and drop it in to a section of the new iTunes window entitled "Artwork." A thumbnail of the artwork will now appear in the box. Alternatively, you can copy the image from a website by right clicking and selecting "Copy" from the drop down menu and just paste it into the "Artwork" section" by right clicking in the artwork box and selecting "Paste" from the drop down menu.
Click "OK." Artwork will now appear at the bottom left of the iTunes interface whenever the song or album is playing.