SCIENTIFIC pictures are not decoration but knowledge. They support our health, our industry, our search for an understanding of ourselves, our world and the universe. William M. Ivins Jr., who was, starting in 1916, the first curator of prints at the Metropolitan Museum of Art, later said that science couldn't exist without reproducible images. Modern science certainly cannot: it is difficult even to imagine astronomy advancing solely on words and numbers.

Images reign in a wide swath of science. Most scientists, like most of the rest of us, understand concepts and structures more quickly and thoroughly when they can be visualized; after all, the brain evolved to deal with images rather than abstractions. Some scientific problems cannot even be solved without images. Now digitization and other transformations in image-making technologies are revolutionizing certain sciences and slyly inserting the issues that dog more commonplace photography.

Brilliant and convincing pictures have a kind of celebrity power: they help attract good minds and public support to sciences that produce fine displays. Astronomy is a good example; millions of people pour over the Hubble and Chandra telescopes' revelations from the edge of space. Nanoscience and nanotechnology are similar successes. Dr. George M. Whitesides, Mallinckrodt Professor of Chemistry at Harvard, says that when instruments make structures no bigger than a few hundred billionths of a meter visible, investigators ''feel immediately comfortable with their existence.'' This has contributed to a kind of express-train development of nanoscience, while chemistry and nuclear physics, lacking such mind-opening imagery, have tended to stay on more local tracks.

To many of us who are not in the sciences, pictures like the Hubble images or the Visual Human Project have seemed like the last refuge of photographic ''truth'' in the current flood of image doubts. A three-day conference at the Massachusetts Institute of Technology in June -- ''Image and Meaning: Envisioning and Communicating Science and Technology'' -- almost sneakily suggested that there is no refuge, that scientific images are more subtly but just as surely subject to interpretation, deconstruction and manipulation as the nearest billboard.

Stunning pictures on science pages, though most likely not lies, are not necessarily absolutely faithful copies of molecules or macrophages or quasars. For one thing, such images may have been ''improved'' to be closer to more closely render the information that was actually there but not entirely caught by the recording camera or other device.

Felice Frankel, an M.I.T. research scientist (and organizer of the Image and Meaning conference), has had to add a color to certain photographs when the photographic instrument did not pick up a tint she could see with her own eyes; the manipulated image was more faithful to visual observation than the unmanipulated. Illustrations may be purposely simplified to make them easier to understand, especially for the lay public. Some published X-ray images of the stars have been adaptively ''smoothed,'' a technique that replaces the value of the intensity of each pixel with the average value of its neighbors, making details easier to see.

Miraculous as microscopes, telescopes and CAT scans are in their capacity to exceed human vision, human beings still have to interpret the results, and human beings have been known to make mistakes. When the Hubble sent back a picture that was said to show a newly discovered planet, the press jumped up and down. A year later, the planet proved not to be a planet after all, but the photograph was exhibited with other scientific photographs at M.I.T. during the conference, and the label still identified it as a planet.

Instruments can't see everything anyway. They make their own adjustments to what they do see, and then the algorithms that convert digital data make other adjustments, so even if the observation didn't change what was observed (and Heisinger assured us that it did) it may change the way it looks. An object looks very different under different modes of observation; consider an X-ray of Uncle Harry's skull compared with a portrait of him. The Crab Nebula changes shape when ''seen'' by instruments using different parts of the electromagnetic spectrum: it appears bell-shaped to X-ray observation, as an irregular cloud under optical examination, and as an even more irregular cloud in an infrared picture. When you're asked to draw the Crab Nebula on the final exam, what will it look like?

Medicine has been turned inside out by new imaging techniques. At the M.I.T. conference, Dr. Richard Satava of the Yale University School of Medicine predicted that one day we will be able to have holograms of our entire inner space printed on a card the size of a credit card. He already does. But John Abele, founder-chairman of Boston Scientific, a medical device manufacturer, says such images, like other medical images, contain extraneous artifacts. Not that a tea kettle shows up where your heart should be, but a tumorlike spot might put in an appearance. Radiologists know this; their business includes telling spots and tumors apart. Mr. Abele, noting that medical imaging is improving, advises that you wait as long as you can to get sick.