A process for producing and fixing an image uses an imaging medium comprising an acid-generating layer or phase comprising a mixture of a superacid precursor, a sensitizer and a secondary acid generator, and a color-change layer comprising an image dye. The sensitizer can absorb radiation of a sensitizer wavelength which does not, in the absence of the sensitizer, cause decomposition of the superacid precursor to form the corresponding superacid. The secondary acid generator is capable of acid-catalyzed thermal decomposition by the first acid to form a second acid, and the image dye undergoes a color change upon contact with the second acid. The medium is imagewise exposed to radiation of the sensitizer wavelength, thereby causing the sensitizer, in exposed areas, to decompose superacid precursor with formation of the fist acid. The medium is then heated to cause, in the exposed areas, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the second acid. The components of the acid-generating and color-change layers or phases are mixed so that in exposed areas, the second acid causes the image dye to change color. At least the non-exposed areas, a copper compound and a reactive material more reactive than the image dye to the products resulting from decomposition of the superacid precursor in the presence of the copper compound, are used to decompose the superacid precursor and fix the image.