Decorating your football invitations sounds too girly! What you're going to need to do is slap some decals onto your football invitations if you want to be taken seriously. And instead of defaulting to free clipart, why not try a few high-quality Photoshop brushes? These will give you a professional quality invitation without the cheesiness factor that you get from clipart that hasn't been updated since 1995. There are a few great options available for download. These include silhouettes of football players, sporting equipment, and of course: team logos! Here are a few to get you started.
NFL Team Logo Brushes
NFL Team Logo Brushes by philipkurz on dA is a great set of all the NFL team logos, plus a bonus of the NFL Logo, NFC logo, AFC logo, alternative Bears and Bengals' logos, and the Houston Oilers logo. Incredibly high quality and a nice addition to any football fan's arsenal.
Sports Photoshop Brushes
As always, redheadstock has done it again. This time she's provided us with a great mega-pack of sports brushes. This is a pack of 49 brushes that are guaranteed to work with Photoshop 7+, Photoshop Elements 2+, GIMP 2.2.6+, and are large high-rez brushes perfect for not only invitations, but printwork in general. Several football brushes are included.
If you use, be sure to credit redheadstock.deviantart.com.