The main reason supermassive black holes aren't visible in images of our galaxy is because of all the dust and stuff in between the center of the galaxy and us. The Milky Way's bulge is ~16000 light-years thick. There are millions of stars, and huge clouds of dust in between us and the black hole, which block visible light. Most of our measurements of the center of our galaxy are made in radio wavelengths.
In addition, compared to galaxies, black holes are small. A supermassive black hole might have a radius of up to 400 AU (for the really large ones). The nearest spiral galaxy to us, Andromeda, is 778,000 parsecs away. This corresponds to an angular size of 0.0005 arcseconds. Even the Hubble Space Telescope only has a resolution of 0.043 arcseconds in the optical. That means that Hubble can only see things that are 100 times this size. So we wouldn't even be able to see any black holes in another galaxy.
That being said, we can make observations of black holes using indirect means. For example, one of the ways we are able to precisely measure the black hole in our galaxy is by watching stars orbit around it. We take pictures of these stars in the infrared and radio wavelengths, which pass through the dust. By measuring how fast these stars are moving, we can calculate the mass of the black hole.
In other galaxies, we can observe other effects of black holes, such as active galactic nuclei, and quasars.
So despite being unable to get an optical picture of a black hole, we still know that they're there.