How far are you into ripping?
If you've just started I'm pretty sure that 'Exact Audio Copy' will find artwork as it rips to Flac. Google Exact Audio Copy to get to there website, click 'download' on the left menu. Make sure you download the right file, as normally there are a few adverts on the page saying 'DOWNLOAD NOW!!' which will download the wrong software.
If you have already ripped a bunch of CD's you can try some of the free music manager software such as Winamp, or Media Monkey. Just get the free version, load it up, point it to your library, and then try and find the 'album artwork' setting. Do a test on a couple of rips to a) make sure it works b) the album artwork is still there when you uninstall the software (assuming you don't want to keep it as your music mananger).
Edit: I use Exact Audio Copy (EAC for short), and it works really well, not frills, just a good bit of software.