A blueprint is generally defined as a plan or technical drawing usually documenting an architecture or an engineering design. Within the world of 3D a "blueprint" can be more generally defined as any plan, its the 2D representation of the 3D model you wish to create. This 2D drawing can be anything from a building plan to a model sheet of game character.
Now what do you do if you lack the skills to create your own "blueprint" or need some obscure blueprint and can't find it? Have no fear, Blendernation is here!
Online you can find hundreds of resources to use as your own blueprints. The first and possibly most obvious source is searching google images for blueprints. Remember no matter what the source, its good practice to contact the original artist for any copyrights they may have in place.
There are also some focused resources created to locate and download blueprints for free. The following is a discussion on a few of these, and is by no means a complete list of all available resources. Please if you any favorites of your own, share them with us. The first resource is http://www.the-blueprints.com/. The-blueprints.com currently has 14138 available blueprints that range from tanks to the human body. Not to mention Boba Fett.
Two more blueprint resources are blueprints.migee.com, which has an assortment of blueprints and smcars with over 6500 blueprints including cars, engines, trailers and more. You can even find the 60's batmobile.
Some excellent anatomy references can be found at fineart.sk. Some nudity is located at the link so be warned that it may not be work safe.
If you are willing to pay for your reference "blueprints" one resource I'd recommend is 3d.sk. The primary reason I'd recommend such a site is that the images are all royalty free and you are assured of the quality. 3d.sk currently has 65,900 royalty free images that are available for instant download. I've used them on a few occasions and have found it to be quite valuable.
Ok. Well now you have your blueprints and you wish to use them in Blender. Luckily, Gaurav Nawani has provided us with an excellent (yet ultra fast) video tutorial (mirrored here) on how to do just that - the short of it being inside the 3D window > ViewÂ > Background ImageÂ > UseÂ Background Image button, then select you image of choice.