1. Go to Album View and select the album.
2. Go to File > Get Info (or Option + i on the keyboard). You will be asked "Are you sure you want to edit information for multiple items?" and click Yes.
3. Paste previously copied artwork graphic into the the small Artwork window on the Info tab.
If you do this for an individual song file, the artwork window will be on a separate artwork tab and you can paste or click the Add button to browse to where the graphic is stored. But when you do it for an album, there is no separate artwork tab and the artwork window is on the Info tab with all the other metadata as described previously.
You will probably most often search for the graphic in your web browser and you can copy the image directly and paste it into iTunes artwork window. When using Google for example, you can switch to the Images page, then click Search Tools > Size > Larger than 400x300, and look for artwork that has the same height and width. I will usually look for artwork that is in the range of 500x500 or 600x600.
When you copy and paste artwork into iTunes as described above, the graphic is stored as metadata within the file and will be available to other music players, except for .WAV files which do not support metadata. When iTunes finds the artwork automatically or you choose 'Get Album Artwork', the file is stored in the iTunes database and will not show up when other music players access the library location.
In those cases, you can copy the automatically found image from the artwork window and paste it right back again, and it will then be stored as metadata in the song file.
Steve