I have now been able to download most of my Artwork to my iPhone. I have done this by turning off Match on my phone and my PC, re-syncing iTunes between my PC and my phone, and turning Match on my PC and phone back on. I am still missing Artwork for about 15 of my Albums (out of 161). But that is likely another problem. All of my music is imported from CD's, so I'll continue to pursue what's going on there.
I just wanted to close out my posting by letting you know how I resolved my problem of missing some Artwork on my iPhone using iTunes Match. I read in another post that iTunes Match may have a problem with higher resolution Artwork. So it deleted the Artwork from iTunes on my PC and copied in new Artwork with a maximum resolution of 300x300. I then updated iTunes Match and that seemed to have solved my problem. Let me know if anyone has any questions.