Ink selection is to some degree affected by paper choice; for a given printer, the manufacturer (especially if known for photo printing) has likely done testing with different ink and paper combinations in order to recommend the best pairings. The viscosity of the ink, the droplet size, and its chemical composition all affect the rate of absorption into the paper, the quality of the colors once they dry, and the resistance to abrasion and fading. Inks specifically designed for photos produce better colors and resist fading better than standard color inks, but also tend to be more expensive per cartridge. Epson even has an ink that is smudge-resistant and waterproof! Be sure to consider your purpose if you’re printing your own photos- if you’re printing for an album or permanent display, a little extra cost is worth avoiding brittle and faded photos in the future. If the picture is just going up on a bulletin board or for casual display, it doesn’t matter nearly so much. PC World magazine ran an article analyzing discount inks and concluded that, even with “quality” discount inks, the manufacturer inks scored far better on longevity.