Apple decided to remove the album artwork preview in the lower left corner of the sidebar from iTunes11. Many users seem to be upset about it’s removal. While there is no way to bring it back other than reverting back to iTunes10, there is a workaround I have been using for now.
iTunes shows album artwork in the upper display window while a song is playing. Note that the artwork will not show if the window is too small. What you can do is double-click on the artwork. Doing so will bring up the artwork in a separate window.
You can then right-click that window to resize it and get it looking exactly how you like. Then move it to a part of your screen as desired.
Though it’s not exactly the same as what it looked like in older versions of iTunes, it’s better than nothing. You can even minimize the large iTunes window and control your music right from the album artwork.