At the top of Artwork tab it shows all the existing artwork for songs in this folder, usually you would expect there to be either one or zero pieces of artwork. The New Artwork area will be where newly added artwork is displayed, at the bottom are listed your songs in this folder with some of their basic metadata to help identification.
You can add new artwork as follows:
Use Artwork Search to open a Google image search for the album, then drag artwork from here.
Alternatively use other search in your web browser and drag the artwork
Alternatively drag files from your computer or use the Browse button to select a file on your computer..
The artwork will then display in the New Artwork area together with its size
Select the tick box in the Replace column for the songs that you want to replace with this artwork, you can use Select All to select all songs
Then use Replace Artwork to replace the artwork for the selected songs, the Artwork column will be updated.
At this point nothing has actually be saved , this will not occur until you have finished making edits on required tabs and then select the Save button.
If you want to add different artwork for different songs, simply repeat the process selecting different songs in the Replace column each time.