If the image is a result of, for example, a plotting library's output, it should just render as-is.
If it's not, then one simple approach is to write a markdown (%md) cell and include a link to the image:
Of course this requires that the image be available somewhere to link to. You can write images to /FileStore on DBFS, and they become available at a URL in your workspace starting with /files. Note that this isn't exactly secure though, as anyone in the workspace could potentially see that file. In some cases that's fine.
For small images, you could encode the bytes of the PNG (for example) image in base64 and then construct a a "data:" URI with that image data. That becomes a self-contained URI you can render in HTML in a markdown cell like any other image. That's only a good idea for pretty small images though.
