Is it possible to insert an image in comments? if the answer is "yes", then how?
The most you can do is provide a link to the image. Comments are not meant to take up a lot of space, and one thing that pictures tend to do is take up space. However, it's common enough that comments may contain the need to reference images in order to help clarify or expand on the point they are making. So simply linking to the images is perfectly fine.
If you do not yet have the image uploaded online, you can still upload it directly through us. Just open a post editor (if you didn't ask the question or yet write an answer on the page you're commenting on, you'll have one already open at the bottom of the page, otherwise you can use the Ask Question button in the top right), and click on the Image button on the toolbar. You can then upload to imgur through us, and then copy the URL from the post body to use in your comment as you see fit.