"Where can I get free tornado pictures? Your options are limited here because people usually just don't give away their prized photography. But there have been some pictures taken by government scientists in field experiments which are public domain. Here are some which were scanned from the National Severe Storms Laboratory's archive for SPC web use. Brief explanations are provided also:
Alfalfa OK, 22 May 1981, looking NNW. A "textbook" tornado extending from the wall cloud of a classic supercell, with a "clear slot" cutting through the cloud base around the near side of the wall cloud. The slot represents part of the occlusion downdraft, an arc of sinking air believed to contribute to tornado development in many cases. The tornado did damage rated at F2.
Altus OK, 11 May 1982, looking NW. A wide, multiple-vortex tornado, with four or five vortices apparent to the left of the big one. This tornado hit Altus Air Force Base and produced F3 damage.
Binger OK, 22 May 1981, looking SW. "Wedge" tornado nearly a mile wide. This violent tornado produced up to F4 damage, flattened cars before wrapping them around denuded trees, and flung cattle and vehicles for hundreds of yards.
Cordell OK, 22 May 1981, looking N. A gust front from the parent thunderstorm hit the bottom part of the tornado vortex and moved it aside to the right. This tornado produced F1 damage.
El Reno OK, 30 April 1978, looking NE. Rope-shaped tornado with the main precipitation core of the supercell to the left of the photo.
Erick OK, 25 April 1989, looking S. There is no condensation funnel; instead, the inner core of the tornado above ground level is outlined by a sheath of fine dust. The tornado hit nothing but grass and dirt -- nothing which could indicate its true strength; therefore, it was rated F0.
Kingsmill TX, 14 May 1977, looking WNW. Tornado in its dying stage. The candy-striped effect comes from the tornado's passing across alternating areas of sun and shadow caused by broken clouds to its upper left.
Mayfield OK, 16 May 1977, looking N. Large tornado which produced F2 damage. The white streak at upper left is a falling hailstone.
Roff OK, 2 May 1984, looking NW. Another "rope" tornado in its dissipating stage. The dense gray area behind the tornado is shafts of heavy rain and hail. It produced F2 damage.
Seymour TX , 10 April 1979, looking NW. Though massive and well-organized, the tornado crossed largely open countryside, hitting flimsy outbuildings, vegetation and utility poles. It snapped and uprooted mesquite trees, which are deep-rooted and notoriously tough, and was rated F2 by NSSL survey teams.

There are more public-domain tornado pictures from the NSSL archive online at the NOAA tornado photo library. [The "high-resolution" versions may take a long time to download on a modem.] If you want hardcopies for research projects, the best bet is to print public-domain images from a high-quality color printer. Even when using public-domain images, you should give proper credit to the source.
Source: The Online Tornado FAQ by Roger Edwards, SPC